Catch cans didn't do much to keep the valves clean...until direct injection became a thing. (Carburetors and injectors clean off the valves.) Now, if you're talking about keeping the oil away from the plugs or exhaust valves (soot), then that's another reason for them, though admittedly minor.
I added one to my diesel. (Direct injection, like all diesels.
) RAM ecodiesels come with a factory-installed oil separator...and I've found out that my OEM separator is VERY good. My catch can gets about a teaspoon every couple thousand miles.