[Dave Noll]

With dissimilar metals (steel spark plug, aluminium head) there's an electrolysis that occurs. That is what welds the plugs into the heads like in Jims example. Electricians use it for the same reason. The same electrolysis that occurs in the cooling system and can be measured with a volt meter because its acting like a battery. And where an anode saves other things in the cooling system from erosion.