Looking at a few ads, I found claims that they reduce slippage and heat more evenly. I doubt that the effect is huge. It might be more critical for something like a soufflé.

I always prefer my bottom to be unglazed, and try, as @crisw says, to avoid slippage.

wat?

@Aster: I was referring to the physical dishes that the creme brulee is cooked in. Sorry for the confusion.