Theoretically the Horned Hold has two entrances; this is the other one.
There's no good reason that PCs couldn't approach the Hold from this direction (provided they found someone to show them how to get there). However the module obviously doesn't expect players to do that - there's no details on breaking down this door or entering the Hold from this room.
The door, in fact, is a "sturdy double door of iron plate", "secured by a bar inside the room". I think that's supposed to be code for "unbreakable". It's a shame the Duergar didn't install this advanced door technology at their main entrance.
As encounters go, the writers phoned this one in. It's four orcs and an ogre, and it goes exactly as orcs and ogres have gone since the beginning of D&D - they draw weapons and start rolling to attack. The ogre, in fact, "has no tactics", which is suspiciously convenient as the space allocated to this encounter didn't leave room to describe any.
Once again runners are the prime consideration here. Escapees from the Western Guard Post don't stop to back these guys up - they keep moving, heading for the chambers of the Hold's commander. The orcs will flee, too, given a good enough reason, and go provide additional backup to the level 7 elite in the next room.
From a DM's perspective, the significance of this room is mostly that it offers a way out of the Hold. If players rescue the captured slaves without clearing every room of the fortress, this back door is a mere two rooms from the slave pits, offering easy egress without having to mop up the remainder of the Duergar.