Each area will have general enemies, special enemies and locale enemies, as well as bosses. While many areas may not require you to fight a boss, if you wish to clean the area out throughly, allowing you to investigate your surroundings without problems, it is best to take them out.

Taking out a boss will even grant you access to a new area, new weapon, secret info to aide your battles or even the assistance of an npc.

Think wisely before deciding to leave an area occupied.