While I don't see the necessity in wearing a ring in the first place - yet I am a sucker for rings - what always gets me is having a fancy overpriced engagement ring that you're supposed to wear for a short period of time and then a simple wedding ring that you're supposed to wear forever.
You can blame the debeers company for the engagement ring. Just like you can blame Kellogg's for the slogan "breakfast is the most important meal of the day".
Wait, do you know women that stop wearing their engagement ring after they're married? Every married woman I've met (that doesn't have a hands based labor job) wears the engagement and wedding ring. All the women in my family have them soldered together some time after, so they don't independently spin on the finger.
I've known one woman who sometimes wore the engagement instead of the wedding ring but this was very odd and uncommon. I know everyone keeps their engagement ring but hardly anyone ever wears them, let alone on a regular basis.
I am curious what region you're from. I would guess it's outside of the US, but perhaps there's an area that differs from the norm I've experienced.
If you are indeed outside the US, I'm guessing it's the American tradition of engagement rings, without the tradition of continuing to wear the engagement rings. That truly would be a (bigger) waste of money.