When I first got a lathe, I bought a group of randomly selected turning tools from the auction site. These two were a mystery to me and I never tried to use them.

This morning I was browsing through Youtube videos and I came across this one: Making Grooving Boring Bars
Duh. They cut an internal groove, as you might want for an o-ring. Might be useful on a reel where the o-ring is a substitute for a “tolerance ring”, allowing a low precision press fit.
The small one makes a 3/32 wide groove and the bigger one makes 1/8.