When you’re battling in caverns or some region that has many rooms, hallways etc – once you kill monster in the area and proceed through the region – you can go back and the monsters will not re-spawn (until you return to the Map).
Example: If you battle through the Goblin Lair, you eventually will be able to go back and forth within the lair without encountering any monsters. Once you progress into another section of the lair with new monsters, you will have the chance to fight again.
This can be good for healing, taking a break, or waiting for other party members to join you.
Sometimes there may be vendors within the region, where you can go sell items to free up space in your inventory, or buy more healing items or resources to craft new items.