Physical wedding invitations are still tradition such that people may get offended if they do not get one in the mail.
I mean, sure, nobody has to get married. But then you might not get custody rights over your children. It might be more difficult to pick your kids up from school. Perhaps it will complicate inheritance. You won't get the tax breaks. You'll have to explain your relationship every time you're introduced to more traditional people, which might include family, or various other people with whom you'd just rather not get into an extended conversation about your life choices. It might complicate banking, especially in case one partner is survived by the other.
There's a direct analog to this in tech. Sure, you're only going to win by innovating. But you have to do so sparingly, knowing that innovation is risky, expensive, and difficult. Sometimes, perhaps most of the time, it's best to "just use postgres".
There are certain things worth innovating on. Perhaps the exchange of some token item (a diamond) is worth one of them. For me, it wasn't, I just bought the stupid thing, and got on with my life. It's nice, it'll last, and it's a time-honored tradition I'll never have to get into 1000 annoying discussions over. Life is just too short, everybody knows it's a diamond, we're married, great.
Or in the case of a lot of folks like myself, it's worth it to break tradition to avoid jumping into a higher tax bracket due to the marriage penalty [1].
I had a relative that was a jeweler at one point. She's the only person I know who opted out of a diamond engagement ring. "waste of money. there are prettier things that can mean more" Her engagement ring was awesome. I don't know enough about stones to say what it was, but she got compliments frequently.
The formality isn't empty in the sense that it's purposeless: it's done for a reason.
After all, marriage itself could be considered an empty formality, so if you're going to bother getting married you might as well embrace the formalized aspects of the occasion.
The contractual stuff came later, and is arguably the less important aspect.
If you take the first view, then the formality of the ceremony and its associated traditions isn't empty — it's the whole point. If you think the contractual, legal aspects of marriage are the the only things that matter, then yeah, might as well just sign on the dotted line.
Anyway, it's not totally relevant to the main story here, since that was very obviously a "This is the official invitation/confirmation" thing, but I think it's worth considering that if you are going to buck tradition, you should try to be absolutely clear that that's what you're doing so that people can adjust.