Without going too much into the origin of the good ole apostrophe, it is an essential part of the English language, and I get annoyed by people misusing it. Apostrophes denote that letters have been omitted, and the word containing an apostrophe is either a contraction of two words, or a shortened one.
The most abused contraction is It is. Which of course gets turned into It's, if used properly.
Cue the possessive its. Example: The cat licked its fur.
Now for the confusion: It's clear that the cat has licked its fur. Some people will jumble up the apostrophes and write Its clear that the cat has licked it's fur. Cue some pretty sulphurous remarks from this writer.