This is what it's all about, dwarves. True dwarven industry in it's purest form. Not only is it a source of rock, stone, ores, and gems, but this big pit is the secondary entrance to our fortress!
The minecart tunnel leading inside might seem like a bad thing to leave open during a siege, but when it's full of cage traps, corkscrews, spears, spikes, and more, it's a one way ticket to hell for anyone dumb enough to go in against the dwarves.
Screenshot taken with stonesense, a dfhack built-in
Don't mind the gore- that's just the pit's other use, and what we did with the survivors of that first siege. Spikes, a long fall, and one angry dwarf are always a winning combination.