To answer the question: I'd say the ability would be something like dropping napalm from a plane, without a plane. It would also be nice for cooking and warming up the campsite, I guess. It could allow a single wizard to stop an oncoming charge, defeat a cavalry charge or beach landing. Burn a village to the ground from atop a hill or tree. It might offer a unique escape: let the wizard climb a tree in the forest, and then burn everything around him (but not his own tree) to kill or drive off soldiers pursuing him in the forest.
ELECTRICAL: Both the brain and heart are absolutely dependent upon finely controlled electrical impulses; which are typically self-regulating. This is why a defibrillator works; the shock forces a new electrical state which a heart can self-regulate back into a normal rhythm. Control of electricity allows a wizard to cause anything from instant unconsciousness, epileptic-like seizures, heart attacks or just instant death, by disrupting or overloading biological electric impulses. Also, of course, disrupting or destroying any electrical device, just like lightning can, and (like an electric arc welder) it might be used to melt or deform metal.
PLASMA: not very useful, but hot enough to burn through metal and stone, perhaps.