Mock Test is an examination where the marks do or do not count, which serves chiefly as practice for future exams or exists so that the teachers are able to set a grade before the end of term. It has several purposes, some specifically for the student and some for the teacher's benefit as well: It gives the student a good idea of what to expect, and useful practice in time management.
So whilst your mock exams and their results don't have the same impact on your future as your summer exams, they are still important. They prepare you for what is to come, and without preparation, your summer exams will be more stressful than they need to be.