When I GM, I often do so in my rec room, sitting in an easy chair, rather than at a table. At such times, rolling large amounts of dice can be problematic, particularly with systems that use a dice pool. With most systems there is typically some mental activity required to interpret the dice (looking for sets, checking how many are over a threshold, adding them all up, etc.) that can be time consuming and/or taxing when one is running - for example - a combat with a large number of NPCs. It's fine for the players - they only need to keep track of one character - but sometimes the GM can use a bit of help.