Well there are numerous reasons why you might need to pay for a usenet provider, it might be that your ISP doesn't provide newsgroups within its service (although most do but not all) or the newsgroups you do get from your ISP may be very limited to what groups you can use, some ISP's have terrible retention (files only stay on a group a day or 2) and some have awful completion (files incomplete) or it could be that your download speed is capped. But the 1 thing nearly all ISP newsgroups are not favoured for is posting to groups. (at best you will receive a threatening email from them with a stern warning and at worst they cancel their service with you.) So if you intend to make any posts don't use your ISP account (unless you know for sure they don't mind)
This is not to say that all ISP's provide poor newsgroups infact some are very good (especially some dutch providers so i'm told) but even if you have a good ISP it's still worth having a premium provider for the odd fill (missing file) which invariably happens from time to time. For people that have good service from their ISP some premium providers offer a 1 off fee for a set number of GB (such as Astraweb $10 for 25GB) a great way to get the odd missing files your ISP misses or to grab something from a group your ISP doesn't provide. So these kind of offers are ideal for you as you don't have to commit to monthly payments.
For anyone else who has poor service from their ISP then a monthly payment scheme is a better option (depending on how much you use usenet though)