It depends if your in school or college.
Both will use them to predict your pass mark for the actual exam, so if for some reasons you don't get that mark they have a good case to put you forward for a remark.
In college it really doesn't matter that much as long as you are putting your homework in to show you are improving over time, and thus the teacher is happy for you to go into the exam. However, in some schools they wont put you into the exam unless you pass the mock, well they did in my day but that was a far while ago, hopefully it's changed now.
One thing to note is I went up a grade between mock and exam, as did some others in my class.
It does give you a good idea on what to expect from the exam and you can now revise harder to make sure you sail through it later on.