Having a ballpark idea of how much fuel a boat uses BEFORE buying it is a good idea.
NOT running out of gas. Gas guages aren't very accurate. Some don't buy the GPH thing, but when I went to the gas station, I could look at my hr. meter and know within a few gallons how much to fill.
Determiing optimal cruising speeds.
Smartcraft or Diacom software is great for these purposes.