We run offshore 3 days at a time. We spend twice as much time trolling as running hard, so the gph varies quite a bit. I always have a pretty good idea of consumption, but it is very reassuring to be able to glance down and confirm the burn. They're also a good indicator of certain problems, like the time I had a fuel line break at the injector and my gph went from 20 to around 70 on that engine as fuel poured into the crankcase.
We also carry a 300 gal bladder, and they come in very handy in determining when I have enough space in my tanks to do the transfer. I feel that they are essential to the way I boat.