Of course there are the legal benefits people have mentioned, but I think a lot of it is that humans love our symbolism and ceremonious tradition. Sure, the ring and ceremony aren’t what make your bond, but it is symbolically immortalized through them.
Weddings don’t have to be incredibly expensive. My wife doesn’t like diamonds, so I proposed to her with a piece of her birth stone. We had our wedding in a state park, we were able to reserve a large section of it for sub $100. We went with simple silver wedding bands from a local jeweler. The biggest cost was food and drink for 70 people. Even though ours was comparatively cheap for a wedding, you could do it way cheaper. Some couples choose to elope, some have a smaller ceremony with only their closest family. It doesn’t need to cost an arm and a leg to be a beautiful day.
CDs are geat, still burn them all the time. I have a Jellyfin server that hosts my digital music collection, but sometimes I may be going on a long drive without internet and CDs are unmatched for that. No battery, no internet requirement, and hold hundreds of hours of music in a a small book in my backseat.