first, as this is an American prison you will probably want to be using the money on your accounts to buy food that to eat instead of depending on the often inedible, non-nutritious stuff that is available from the prison kitchen.
second, it's quite a well known scandal regarding how systems are set up to rip off inmates for phone services https://www.prisonlegalnews.org/news/2015/jul/31/inside-shad... so if you are a prisoner with money that is nice that you can actually call people when you want as long as you want.
third, depending on where you are at the commissary probably has other things you want, and if you have enough of those things you can trade that for other things inside that you might want.
(of course you do not legally access the money, it is on an account that you can use, you can also - although the process can be convoluted - release money to visitors and those visitors could give it to other people who are related or confederates of other people inside allowing you to buy other things that way [which is of course going to be against the rules])
I could go on, but I find the question somewhat weird - I'm guessing you must be from a country with no knowledge of American pop-culture or anything because this is quite a bit past even 10000 territory https://www.xkcd.com/1053/
