This gun is a big hefty fusion reactor for a cannon. It takes a while for it to charge up and fire, but when it does, it fires a huge fusion ball that can take out an army of small demons. The biggest drawback would have to be that it comsumes a lot of energy charges, use it when you only really need it.
The BFG 9000 is particulary useful for killing Arch-Viles, due to its ability to quickly dispatch them at close range with no splash damage, as well as Spider Masterminds and Cyberdemons, both of which otherwise require over a dozen rockets to kill. It is also highly useful in tight situations when the player is being assaulted at close range by a swarm of medium-level monsters.