Since an attacker must have double the number of steps of the VALUE OF THE FORTRESS I am not sure why there would be any value for a defender to enter a fortress. It seems like it would make it easier for the attacker to destroy the units while they are inside the fortress. How do defending units inside the fortress add to its defense? Not seeing it in the rules....am I missing something?
One reason would be for inferior force to delay the capture of a province and/or avoid or delay its own elimination. For a larger force that isn't clearly inferior there's little to gain and pitched battle may be preferable.