Mocks are quiet important in my opinion. I realised their advantages when I went to check sixth form applIcation form and saw that for them to give you unconditional offer, they want a pretty well results in gcse's mocks.
Mocks are a way to get a sense of what its like in the actual GCSE. Although they don't count towards anything, they are able to help you assess in what you are stuck on (content) and what you need to do more revision on.
It also improves your time management as if you think you didn't have enough time to revise, then before the GCSEs you'll know you need to prepare way in advance.