I see used USGI groin carriers on ebay, but I've never seen the plates that go in them. I've been aware of groin armor for a long time, seen it used by the .mil. I've always thought if I had armor I'd want my groin armored, if I could only have one piece of armor it'd be groin armor.
Eta, like this.
https://www.ebay.com/itm/Eagle-Industries-CIRAS-Vest-Groin-Protector-Carrier-Cover-multicam-OCP/311635860576?hash=item488ef1dc60:g:iGsAAOSwB-1YuzRU

The old name for this would be a Cuirasse or Fauld plate(s). I always wondered how much they would get in the way getting in or out of tight vehicles like an uparmored Hummer or an MRAP. In the old days, they were split for riding on horses. The solid piece in front was for ground/foot combat. I think I would probably like a steel or ceramic plate in position, backface deformation really doesn't sound like fun.
As for the reason to wear them:

I can't recall any USGI plate armor below the belt, at least to regular folks. Just dickflaps for the IBA/IOTV and later the combat diaper. I've seen a few lower ab plates on the open market, no groin though. UARM makes one. I'm sure there are more.

If you've never worn something like this before, make an effort to try before you buy; they're very uncomfortable.
Many long, long days logging mileage while wearing a combat diaper made me wonder if keeping my giblets was worth the suffering.

The us military version is made out of Kevlar / is a " soft armor insert" thats flexable like a kevlar vest.