Both of those, but mostly the latter. They can also help determine the required contrast for a negative -- you meter separately off of the highlights and shadows, and use a little dial/calculator thing to determine the contrast grade.
Originally Posted by jnanian
At least, that's the theory. I've used a couple of enlarger meters, and I've found that they steer me wrong more often than they steer me right. Maybe mine are dodgy or maybe I'm using them wrong, though. Either way, I've stopped using them.