A good comment from back when Reddit was good:

It’s what’s called a dummy pronoun, a pronoun that carries no semantic information and is only used to fulfill a syntactic requirement. More generally that’s called a syntactic expletive, although that page says that there’s some argument about whether this particular use of a dummy pronoun falls under that category.

This is a common construction in languages that don’t allow dropping pronouns (non-“pro-drop” languages). German has “es regnet”, French (the only Romance language that isn’t pro-drop, IIRC) has “il pleut”, but in Italian it’s simply “piove”.