Sometimes I think we should just be done with the damn things. But they do help us distinguish between possessive singulars and possessive plurals. I got so many questions about apostrophes that I made up this canned answer to send attached to specific questions.
An apostrophe is used to indicate missing letters. This is easy to see in contractions (can’t, won’t, didn’t, etc.)
About a thousand years ago we used to indicate possession with “-es” at the end of a noun. But language changes over time. Today we have dropped the “e” and we use an apostrophe to mark the missing letter.
The dog’s house. (one dog having one house)
The dog’s bone. (one dog having one bone)
The dog’s bones. (one dog having many bones.)
This would be easy enough, except we also use “s” to indicate plural nouns.
The dogs run together. (more than one dog)
The dogs play together. (more than one dog)
So what do we do when we have a plural noun which is also a possessive?
The answer is we put the “s” in first to indicate plural and then we put the apostrophe after to indicate the possessive.
The dogs’ bones. (bones belonging to more than one dog)
Really you can see that the “s” first indicates plural. We might have written “dogs’s” to indicate plural possessive, but that would be awkward to say, so we drop the final s in both writing and pronunciation and just leave it as dogs’ which is easier to say but causes infinite problems to those learning grammar.