Listen up, journalists of The Telegraph, The Guardian and others; prick up your ears, pedants of the Apostrophe Protection Society: your holy squiggle is neither grammar nor punctuation. It’s orthography and, as such, a convention — and a fairly inconsistent one at that, like most English spelling.

© David Clarke/Solent News & Photo Agency, via The Times

Grammar is mostly concerned with syntax and morphology, while punctuation mainly stems from the need to represent pauses in speech, with the sometimes added preoccupation of indicating different types of relationships between phrases or clauses.

Rule n°1: the apostrophe marks an elision: ‘it’s’ for ‘it is’, ‘you’ve’ for ‘you have’, ‘B’ford’ for ‘Bradford’, etc.
This is why there is no apostrophe to mark the plural, as nothing is elided there. Or is it? As it happens, usage varies with acronyms (‘MPs’ or ‘MP’s’) and numbers (‘the 1980s’ or ‘the 1980’s’).

Enter the genitive (or ‘possessive’): Rule n°2 is that the genitive ‘s’ should be preceded by an apostrophe, which… contradicts Rule n°1 since nothing is elided there either, at least in contemporary English.

Matter of fact, the genitive is the last example of case marking on the English noun. It comes all the way from Old English, where ‘s’ or ‘es’ was just affixed to the noun, without any separation of any sort.

The apostrophe popped up much later, in the 16th c., and while it may be said that it stood for the missing ‘e’, its use remained largely inconsistent until well into the 19th c. (cf. Mugglestone (ed.), The Oxford History of English, 2006).

Excerpt from The Tempest Act 1 Sc. 2, from Shakespeare’s First Folio (1623)

What’s more, the apostrophe magically disappears from the genitive determiner and pronoun “its”, contradicting Rule n°2.

So not only is it neither grammar nor punctuation, but it’s a mess that no one in their right mind would ever claim to be ‘logical’ or ‘necessary for clarity’.

To conclude, don’t ever ask me this — I might be rude: