From The Aryeh Kaplan Anthology II pg. 326

Another allusion to removal of Niddah status by water is in the Torah, "It shall be pure only if it is purged in water for a Niddah." (Numbers 31:23). According to the Talmud (Avodah Zara 75b, Tosafos loc. cit.) this indicates that vessels must be immersed in a Mikvah just like a Niddah.

The immersion was well established as part of the Oral Torah; the Prophet Zechariah (13:1) takes it for granted when he says, "On that day a fountain shall be opened for the house of David, and the inhabitants of Jerusalem, for purging and for the Niddah." We see a clear established fact that a Niddah required imersion for her purification.

Mikvah is one of the most important aspects of Jewish married life. In a sense it is even more important for the marriage bond than the wedding ceremony.

It can be seen as a renewal of the couple's marriage.

to be continued