Yes, the formalisation in the form of the signing of a legal contract is the essence. Cost is an entirely different aspect of it. But some people love the attention, the pomp and circumstance, and all the (imho unnecessary) rituals. Personally, I don’t like rituals and the attention exposure they bring. But to each their own, so if people really want to go through all the faff, then by all means, do it.
The real problem, as I see it, is the misguided idea that a couple is not getting “properly” married unless they also add the extra stuff with supernatural blessings and rituals, throwing a big party, etc. It’s as if the couple bereaves their friends and relatives of a party they are entitled to if they do it discretely. Same with infant baptism - I know lots of otherwise secular people who let their babies go through that because their parents and/or grandparents expect it and become disappointed if they don’t do it. Because rituals and shit.