The reasons are to protect the operating entity from the risk inherent with operating equipment. If your tractor trailer rolls over and kills a family the transportation company is sued and potentially bankrupted but the operating entity has some protection. Next time you see a Wal Mart truck notice it's from Wal Mart Transportation Co, Inc. or something like that. That's not Wal Mart. Run it like a real business. It owns the assets, it carries insurance, it hires drivers and carries the payroll, etc. You can't create the entity and park assets there but do everything else through the operating entity. It has to look like a separate business. This isn't one that's about taxes, it's about risk management.
By the way you should do the same for any real estate the business owns and uses for the same reasons. If someone gets hurt and the operating entity gets sued your real estate isn't exposed to the suit.