The LSAT is, simply put, the dominant factor in law school admissions. It is generally weighed 2-4 times as heavily as your undergraduate GPA, and is usually worth 60% to 80% of your total application in the eyes of law schools.
To get into any law school today, you generally need an LSAT score near or above average, and to get into most top programs, you usually need a score above the 90th percentile. (To get into the truly national schools, you'll often need a score above the 95th percentile.)
The LSAT is the most important test you will take as a lawyer. It determines if you will get into law school, and where you can study.
It can also easily earn you over $100,000 in scholarship money.
For both reasons, it is essential that you are fully prepared before taking the LSAT.