Depends on school, my school has entered me for AS examinations for all my subjects. Some might enter you in only for subjects you intend on dropping so you get something out of it, others give you end of year mocks.
I take it you're part of the latter group. Universities typically look at these but they'll tend to look towards predictions and GCSEs slightly more than they would with say an AS examination which gives a more concrete indication of progress. It also depends on the school, some schools I know are a little bit more liberal with their handling of the exams, at least by marking etc.
Ok, let's assume you've not done so well in your end of year mocks, AS, etc.
You probably may get your predicted grades still, at least sufficient to apply some places you want, if you feel that your grades are low and haven't gotten an offer, you can buckle down for the final exams. You get the grades you want, you then can either go through clearing. and see what's available (usually when uni spaces aren't quite full and they're trying to get you spots), alternatively, take a gap year out, so you can reapply for the next cycle.
There are some perceived 'advantages', here. Like I said, a lot of applicants are applying with very vague mock grades and predicted grades, which have a much larger margin for error, your main pro here is that if you have extremely good grades, you have ALREADY achieved them, they are banked and secured. Meaning should you possess the grades, there is a higher chance (not guarantee), of getting successful offers.