I've seen it and done it, but it is rare. The reasons you list are about correct, but the ones I have seen simply have a small fuel tank and room to store the most oft used machines, with the equipment maintenance done in the main building. In other words, the sheds save daily travel time just to get where the greens mowers, etc. need to be. The mechanic can probably drive to that outlying shed with a selection of hand tools when necessary.