If you’re looking for a rational argument for the big party or the religious ceremony or anything like that – You won’t find it. These things are meant to play to the emotional, and this isn’t a flaw, it’s the whole point. People really need to embrace that we are, in fact, very emotional creatures, and that this is not a bad thing, and that yes, a lot of the things we do are done just for the emotional satisfaction of it. Because it’s fun, because it will make you or someone you care about happy.

If neither you nor your partner give two shits about big parties or ceremonies, then neither of you needs to bother. If said partner does want this and you don’t, then y’know, maybe have a good chat about that and find a compromise. That’s how partnerships work. (Me personally I’d love to organise my own wedding and go all quirky with it, but I can live without it)

Being legally married is a separate thing, and is inexpensive in most countries (just a small fee so the bureaucrats can process the bureaucracy), and at least in my country is often done weeks if not months in advance of the big party and/or religious ceremony, with the couple already being legally married while they organise their wedding stuff. To be legally married is to have you and your partner recognised by The State ™ as being a family unit. This has uses for a few situations in life.