Main reason is because we want to. Like Mickey says ,sometimes you want to blow the crap out of something. The fact is that most of my bigbore shooting (and I suppose it is the same with a few others out here,)
is done at charging ant hills/ termite mounts or just plain old paper. It is like asking why people want 600HP cars or 160HP motor bikes and so on.
As for serious hunting, Ive never gone after dangerous game,YET. When I do ,I would rather have a good 458 Lott with me than a smaller 375. I know lots of great hunters get by with a 375 ,but I would be more comfortable with a bigger hammer in case my bullet placement is not as good as theirs. If it starts running quickly towards or away from me ,then a bigger bore will be handy there. My 458 Lott will be more dual purpose, using 300 GN barness bullets which are the same BC as a 180gn 308 bullet and going at about 2600-2700 fps ,should have a similar trajectory to a 30-06. This should be good on pigs,camels,scrub bulls,sambar deer ,then go to Africa and be used as an only gun for Wathogs to Zebra,Wilderbeast and Kudu. If a Cape Buff pokes its head out ,then I can slip some 500 gn bullets in the mag. Big bores can realy be versatile.