Lots of room.
The area behind the rear seats is fairly spacious; I can fit a week’s worth of groceries in there with plenty of room to spare.
But sometimes you need even more cargo space. Sometimes your husband will look at the decades worth of accumulated computer debris in the basement, and announce that “You can get rid of everything on this wall from here to the door.” And when he says that, you want to do it fast, before he changes his mind. So you need enough cargo space to take six computer monitors down to the city recycling facility, or eight to ten baskets full of power cords, ribbon cable, PC cards, etc. down to Purple Elephant. You want to be able to do these things in one big trip.
The back seats of the Fit will fold down completely flat. This gives me plenty of room for hauling all that stuff.
There’s also a surprising amount of head room for such a little car. I used to drive an Acura Integra; I was perfectly comfortable in it, but when my 6’2″ husband rode with me his head would brush the roof. In the Fit he’s got plenty of headroom—one of the reasons his car is also a Fit.
And as a side note, I’m quite gleeful about getting the large amount of crap out of the basement.