There were slides from the show that made the gimmick clearer.
PrimeMasters hide in shells that become weapons. In robot mode they ride Legend class vehicles.
The PrimeMasters transform into power cores that can plug into deluxes and voyagers in a similar way to Minicons could attach to Bulks in Armada. (It appears they may also trigger a gimmick in Voyagers).
They can also replace the matrix of the leader class toy.
For deluxes, voyagers and leaders the PrimeMasters grant a "special power" to the larger robot (but like the Titan Masters and their special powers it's just text not an actual thing except Voyagers maybe)
For backward compatibility the three deluxes (so far) are also combiner limbs and the voyager has what looks like combiner limb connection points.
The power cores can also take the place of a Titan Master on Titans Return toys (giving the Transformer a head very reminiscent of a Celestial from Marvel comics).
And thinking about it plugging these "cores" into Overlord's chest make more sense than those pics of him holding other heads there.