There are lots and lots of different ways and reasons to use a tractor.
A few people have a sort of 'day tractor', moveable pen just for daytime use and the chickens are returned to the coop every night (by hand, by moving the tractor, or by training them to come to food)
Most peoples' tractors are for 24/7 occupancy, like a very small coop-and-run that can be moved around the yard. Some people do this to let the chickens 'work' fallowed garden beds; some do it in hopes of improving the lawn (it didn't improve MY lawn, but, circumstances vary); some do it to give chickens a constant supply of fresh grazing ground; some do it to decrease sanitation challenges, either for pets or layers or (especially) meat chickens; some use tractors because zoning forbids a permanent structure to be constructed; some build a tractor because they're not initially sure how many chickens they want or what they'd want to build as a permanent coop and the tractor offers a cheap pro-tem solution while they mull over long range plans. I'm probably missing some other reasons.
And, not everyone uses a tractor year-round -- I'm not actually using mine at all this summer, but even in past years when I did, the chickens all went into a real building for their winter housing.
So, lots of possible reasons
Pat
A few people have a sort of 'day tractor', moveable pen just for daytime use and the chickens are returned to the coop every night (by hand, by moving the tractor, or by training them to come to food)
Most peoples' tractors are for 24/7 occupancy, like a very small coop-and-run that can be moved around the yard. Some people do this to let the chickens 'work' fallowed garden beds; some do it in hopes of improving the lawn (it didn't improve MY lawn, but, circumstances vary); some do it to give chickens a constant supply of fresh grazing ground; some do it to decrease sanitation challenges, either for pets or layers or (especially) meat chickens; some use tractors because zoning forbids a permanent structure to be constructed; some build a tractor because they're not initially sure how many chickens they want or what they'd want to build as a permanent coop and the tractor offers a cheap pro-tem solution while they mull over long range plans. I'm probably missing some other reasons.
And, not everyone uses a tractor year-round -- I'm not actually using mine at all this summer, but even in past years when I did, the chickens all went into a real building for their winter housing.
So, lots of possible reasons

Pat