Try to take it seriously because it will help you in your real exams and it will help you identify your weaknesses. Also my teacher said that it can affect the grade they predict you for your A2. For example, if someone gets a B and 1 mark of an A at AS and all his mock exams are Ds/Es then they will not predict him an A for A2. But if he's worked hard throughout the year and got As in all his mocks but got 1 mark off at the exam, they will be more likely to predict them an A. Of course, if you get an A at AS anyway it won't make too much of a difference.