I occasionally hear Centre reporting me as unverified-altitude traffic to IFR aircraft. I usually pipe up and say I’m at X location at Y altitude on Z QNH. Centre usually asks me to squawk ident. Presumably Centre can then ‘trust’ the altitude from my transponder as verified and disregard me (or otherwise) as a risk to nearby IFR aircraft.
I also occasionally hear Centre broadcasting to multiple VFR aircraft at a particular location, simply to warn of their proximity to each other.
These interactions are probably not necessary, but they certainly cannot hurt so far as I can tell.
The one that I always get a giggle at - because I’m sure it would give Dick conniptions - is when a VFR pilot makes his or her perfect inbound call on Area rather than CTAF, and Centre pipes up and tells the pilot that s/he’s on Area rather than CTAF...