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First I would like to point out that each style/school/time period had its own 18 classes of weapons, so I don't want any "thats not what my teacher tells me messages". If yours is different then you can assume that your teacher/style has training and information to support your variation.
Traditionally the weapons were taught as tactical and military applications. Each weapon has a range to dominate and a range to avoid. Angles of attack, weakness to take advantage of, and strengths to build and focus. While an older school would have many more than 18 weapons, they would be divided into 18 classes for the purpose of training efficiency and to advertise a complete military system. Eighteen classes would cover the weapons used by all ranks of the military and the various formations of orthodox and unorthodox military units. After a basic version of the weapon classes were taught, a student might learn whatever variations which that school maintained and focused on. Some variations were indoor versus outdoor weapons, male and female weapons, spiritual weapons, bandit weapons, pirate weapons, escort weapons, bodyguard weapons, weapons from a specific historical period, and court weapons.
For example the indoor versus outdoor variations would differ in size and design so that they could be used indoors. swords would be shorter, pole weapons would have two ends so they would not have to be spun around, and the weapons would be lighter.
The female variations tended to be much smaller, one handed, and easily hidden. The blades tend to be wider and look ornamental. For example, the female tiger fork looks like a large salad fork and I do not know why.
Spiritual weapons tend to be lighter, such that they will bend and break if used violently. The other direction spiritual weapons go is that they are modified to not cut or pierce. As your skill increases, the weapon is made bigger and duller to keep the user from cutting or stabbing people. Knives end up looking like a brick with a handle or a bit of foil that can barely hold its shape.
Bandit weapons tend to have a shorter handle and heavier blade. All horseback weapons are shorter because horses are expensive and if you had the money to keep a horse, you would not be a bandit. Other weapons have a heavier blade to impress the victims and can be used as a pan to cook food or shovel to dig with. That sword may be all the bandit has so it better do more than cut people.
Pirate weapons are shorter and lighter so that they can be swung on a ship without cutting the ropes and they can be held while swimming. Pirates also had different sharpening concepts. Many military weapons used the bottom third of a blade to block with and kept it dull while pirates would sharpen the whole blade and simply use a second item for blocking.
Escort weapons and bodyguard weapons shared the quality of defending an item or person from bandits and assassins. Weapons tended to be balanced for throwing, the cases tended to be modified for an easier draw, and flexible variations existed which could be wrapped around the body. This was in addition to smaller weapons that could be used in a crowded area. Also weapons tended to have hooks that could catch and disarm other weapons which would be important in a populated area.
Weapons from a historical time period tend to have design varitations which set them apart. Coin weapons had holes in the blade that would have an actual coin in it. The coin could be knocked out to surprise the opponent and the hole used to catch the opponents weapon. Replicas usually have a coin impression but at one time every weapon with a blade big enough to structurally allow such a hole had a coin in it. There was a time when every weapon had a hook on it. Hook spears, hook swords, hook dao, and anything else which had room for a hook. The tiger hook swords are a tiger variation on the original hook sword. Some weapons had large rings on the handle. A school may keep this training alive as the basic or advanced variations on the 18 classes of weapons.
Court weapons tended to be easily hidden because it was illegal to carry a weapon into the rulers castle. The fan, the bladed fan, the thinner chain whips, short daos and jinns that could be hidden in the palm, and various throwing shapes that could fit in a pocket. Many weapons were also considered badges of office and were allowed in the castle even if not allowed in some rooms or within reach of the ruler.
The training would expand the open hand techniques to different applications. The sword hand, chopping hand, and the cutting hand all have different applications even though they look the same. The techniques named for weapon movements would also allow the translation of skills from open hand to weapon or weapon to open hand. Each weapon would have a list of qualities that would be practiced and would explain the language used to name open hand movements. A General cutting would be bigger and more powerful than a soldier cutting. Cutting itself would translate to a sword movement with the corresponding foot steps while a chopping movement would use the shorter axe motions. A military name would signify the opponent wearing armor and the power necessary to overcome the protection while a move named after a farmer would be more basic and use less strength.
The weapons also enhanced other capacities. The weight should make this obvious. The pole weapons tend to build stronger stancework which helps counter throws. Swords build stronger wrists which gives an obvious advantage against wrist locks. Each weapon would have a body part that they would build power in and this would also be used to guide the students if time was a factor. If a weapon needs wrist strength then you would want to teach that to students who already had a strong wrist. If you had all the time in the world or had already taught the easy weapons, then you would use the weapons training to remove a student's weakness.
At one time the hight of mastery was to know "18 classes of weapons" It didn't expand the mind that fast but it was good enough at the time. A fighter would usually retire at some point and open a school/clinic or join a temple and spend his last years in contemplation of the universe.
There was a time when the body was trained to replace all the weapons and people thought that the weapon training was not needed. This did not last long. The truth is that there are some things that only weapon work will teach a student. There are skills that can only be learned by defending against another weapon. This is why all schools teach some form of weapon work. A teacher who says that weapons training is not necessary simply does not teach the skills that require weapons and will leave their students with very large skill gaps.
To the left is a list and each weapon will have a page (if I have the Time) and possibly a picture.