IMO tractors as a day and night living situation sound great in theory, but don't work out so well in reality. They also only seem to work okay for 2-3 hens. I have seen many people start with tractors, then give up on them and build a real coop and run.
Now if you have, or want to have, a permanent coop/run, and add a day tractor to that equation, that could work out well for a small number of chickens. The uses of it--to take advantage of pasture that you have, weed out garden beds, raising broilers, as Pat said. Make sure it is predator-proof!