You only get experience for your character outside the dungeon upon successful completion of a run. The amount awarded like and is similar to the bonus completion you get for a levelling roulette, and as such essentially can fulfil the same purpose. People can, and have, levelled entire jobs via Palace of the Dead. I dont recommend doing this, however, since the pace inside the dungeon tends to be too fast to really learn proper full rotations and the like leading people who gi this route tending to be... let's say less than great at their job outside of it.

You also gain experience for your character inside the dungeon at a greatly accelerated rate, but this doesn't apply to anything outside the dungeon. Your level inside persists as long as you dont reset the save slot, so you could easily be level 60 inside but only level 15 outside.