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. If neither you nor your partner give two shits about big parties or ceremonies, then y’know, 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.

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 is often done weeks in advance of the big party and/or religious ceremony. 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.