I just upgraded from a "5HP"/20g (actually 1-2 HP) portable to a true
4HP/80g two-stage. My reasons were two-fold:

1. The single stage couldn't keep pressure *above* 120, which is where
some of my nailers like to be. 90 psi isn't always enough. The
new one bottoms out at 135 PSI :-)

2. Pneumatic sanders suck up a LOT of air - they're one of the more
air-hungry tools you can get. Went from 4 to 12 CFM :-)

I went with a cast iron oiled compressor this time, but I still need
to pick up an oil trap before I can retire the old one.