Why would a sugar glider eat his feces?

There are some species of animals, including rabbits and (I presume) sugar gliders, who eat a lot of leaves and have a diet high in cellulose, and they cannot digest this diet with just one pass through their relatively small digestive systems (cows can digest cellulose because they have four stomachs, and can retain the food for much longer than a rabbit can). Eating their own feces (or pellets) gives them the chance to complete the digestive process.