The BFG 9000 is a big gun that fires giant balls of green plasma, which usually destroys everything in the general direction that you're pointing the weapon when it discharges. It has a warm up time, a charging time, and a firing time.
In DOOM 3, you take damage from being too close when the weapon discharges, and the weapon can be 'charged' to the point where the gun explodes, killing you instantly.
Because of this, this guns' use is somewhat situational, you can't charge it too pre-emptively else you will die. You can't discharge it in a narrow space else you will die. You can't discharge it with an enemy in front of you or... you get the idea.
When is the best time to use this weapon instead of another gun?
Which enemies need more than one or two levels of charge from this weapon in order to die?