They're only important in some cases:
- if you're applying to a competitive sixth form, you ideally need good mock grades
- if you're applying for work experience, you again need good mock grades
- they show where your gaps are (however this is even dubious because mocks only test usually on one unit in the qualification and each exam paper doesn't test on everything in that unit)
You can fail all of your mocks (or seriously under-perform) and still get into sixth form providing your teacher predicted grades meet the sixth form requirements.
In the scheme of things - no, they do not matter.