It's more about what it says to your partner, commitment through sacrifice and ability to provide

Buy her a house? Buy insurance? Put all your pay into a joint bank account?

If you buy something that's directly useful then you (in the general case) risk being exploited. By buying something that can't be used or resold, you give the signal but in a "safe" way.

Why is signalling that to peers useful or a good thing?

That's been thought out a while ago: https://en.wikipedia.org/wiki/Veblen_good

Makes you feel good.

I suppose you could accomplish the same thing by just not telling people that it's moissanite.

I think most girls check somehow the ring/diamond value.

My bad, I meant the girl not telling other people. I'm not sure it bodes well for the relationship if the guy doesn't tell the girl!