well, it can be anything really, getting your arm in there to sort something with the rock work, remove a dead invert or fish, move a coral, work on any equipment you have in the tank...
The last you want is to be hampered by the hood. When you have a tank that is quite deep and stands high off the ground, you need to be able to access all area's with removing things like lights and hoods else a 5 minute job can turn into half an hour if you have to remove lights out so you can lift the hood off...
All in all, it makes life so much easier...
Niko