ken 120
That compartment is a lazarette locker, and kinda matches the one on the starboard side except it's set up to be closed-bottomed so it can be made into a bait well or live well.
Some non-fishing folks use it to hold the second, or third, or forth battery.
Outside of that, it can be used for just about whatever you need extra storage for: ropes, fenders, anchors, crabbing gear, canvas parts, etc.
It's not always waterproof (depending on your individual boat), so keep that in mind.
Joe.