An apostrophe is a little thing that can cause a lot of trouble. When tutors are complaining about mistakes that students make, one of the things they complain about most often is that students go wrong in the way they use apostrophes.
For many tutors, a student who can't use apostrophes correctly is a student who can't write well. So, getting apostrophes wrong is a serious mistake - you can lose marks for it.
But don't worry! Apostrophes aren't difficult, and it is quite easy to learn how to use them correctly.