Lath and Plaster Framing: a Floating 2x4?
If there is a wall directly below it is most likely the "top plate" that the walls studs are nailed to. If there are no studs attached it then is most likely a "stiffener" for the wood lath underneath it. In this case it would be keeping the lath strips in-line and level in order to prevent bouncing or flex. It has been installed there with a purpose unless you can install a substitute piece to replace the board leave it until you verify what purpose it serves.