There are many reason and everybody has their own but I imagine for many it comes down to
-wanting to house a large showpiece fish such as an Aro
-wanting to house multiple large showpiece species together
-having an aggressive comm. that needs more space to be successful
-wanting to give their fish as much room as they can afford for comfort
-wanting to increase the likely hood of success keeping darty or sensitive fish
-wanting to keep more fish then they are currently capable.
Personally I'd like to have one really large show tank and multiple smaller tanks in the 125-240 range. The largest tank would be for housing Aros and Rays. While the smaller would be either dedicated toward breeding certain species or creating mini copies of habitats of peaceful community fish.
Actually, the reason I keep going for bigger tanks is due to my aging eyesight. I can't see small fish anymore. So...

