Until Kamikazi shows up to answer this definitively, I am going to take a swing by assuming you're talking about what used to be called the Mid Store Charge Card.
At any rate, if everything is somoewhat similar to ages past, what you are given is a "credit card" that you can purchase things with in the Midstore, including uniforms, books, and all the other stuff they sell.
While it is true that, especially as a Plebe, you see practically nothing of your actual paycheck, some of it is "saved" in an account with your name on it. After a period of time (in my day it was two years), you get back whatever is in that account.
So, if you're smart, you can get quite a whopping piece of change back. If you're stupid (as I was), you can go a while without getting paid at all until the amount of your previous charges are paid off. For me it was two months.
So be smart: don't blow your money on stuff you don't need. While I can definitely see you NOT doing this as a Plebe, during your 3/C and 2/C years, you will be "required" to purchase additional uniforms that in many cases you don't need (how many SDB blouses do you really need? Two? They issued us FOUR, and they WEREN'T CHEAP). You may elect to buy your books used, which will save you a bundle. Etc., etc.
Please take this answer with a grain of salt. I am going on memory and a lot has changed, but the jist is still valid from what I've heard. Kamikazi and Jadler (who are actually there now) can provide MUCH better answers.