It’s a shorthand for all those other legal arrangements, in a pragmatic sense. You can build the same thing with documents that confer the different legal relationships, or you can use the pre-packaged bundle. A lot of the one-off arrangements require a lawyer and filling fees for each document, where the bundle can be done for a $25 or so fee, and a judge or the clerk who collected the fee, depending on your jurisdiction.

There are also social and relationship perks to a public declaration of commitment. It doesn’t change anything, but a public declaration can make things explicit on all accounts.
Rings are just a social shorthand to communicate that to others passively

They also don’t actually need to be expensive. They became expensive because people are usually willing to shell out a little more for a special occasion, and a lot of people wedged themselves in and argued that without them it wasn’t really special. If you can’t put a price on love, then how can $10k be too much?

If you’ve decided to make a public commitment, a little party to celebrate is legitimately fun. You just need to separate what you need for the party to be fun and feeling like the scale of the party is a testament to your love or sincerity.

When I got married the ceremony was five minutes and done by a friend of ours, we had our friends and the closer circle of relatives as guests and we didn’t need to save up for things because we only got what would make us happy for our party. Our rings were cheaper than most because we talked to a jewler and had them make something according to our designs, and neither of us like diamonds. (Mine is a metal reinforced piece of a beautiful rock we found while rock hunting at a favorite camping spot, and hers is her favorite color, laid out well to avoid snagging on clothing.)