Two reasons: Practical considerations (shared assets, certain legal protections, I’ve seen people get married for an easier go re: immigration in some cases, etc. Basically check your local laws); and ritualistic.

I find people often discount the importance of certain ritual practices in Western secular society, and for a lot of people ritual in general is a whole lot of fluff and nonsense. But having a ceremony to recognize a formal joining of two people, and by extension their families (to varying degrees), with the at least ostensible intent that you will live and die in partnership with that person, is a powerful thing. It’s a common ritual among multiple societies, with lots of variation and differences in exactly what it signifies, but the ubiquity speaks to that power IMO.

Don’t get me wrong - I think divorce is a good thing for when the partnership truly does not and cannot work, and people can live happily in lifelong unions without marriage - but for some folks, taking that vow in the eyes of your friends and family (and whatever deity concept you may have, if that’s your kink) is a very important and serious thing. Something changes, to some degree, when you take that oath.

It doesn’t have to be expensive - that it often is, IMO, is a function of capitalism infecting a beautiful thing more than anything else. You can have a wedding in someone’s backyard officiated by someone who paid $25 online for a certificate, with a small number of close friends and a potluck BBQ afterwards, and it would be just as valid and meaningful as a wedding that cost 100k (shit, IME the smaller one is actually more meaningful in a lot of cases). It’s the intent, ritual, and meaning participating parties place on it that’s important.