An airtight belly is very important in heat and cooling efficiency and in preventing water lines from freezing in cold weather. However, it is penetrated by the drain, supply and electrical and gas lines. These openings may or may not have been sealed properly. In addition the belly may have been damaged during transport and not carefully sealed again. Cats and mice are not stupid either and love to get inside where it is warm. A mouse can get through a 1/4" opening!!
Mobile home supply places sell belly repair material and aerosol adhesive that is used to attach it. You simply cut a piece large enough to cover the problem, spray adhesive on both the repair piece and the surface it will be attached to, wait for it to set and press it in place. If that sounded real simple, you have obviously not had the pleasure of lying on your back in the dark under a home, breathing adhesive fumes, trying to keep the repair surfaces clean and dry, while you position it properly.
Is it any wonder that most belly damage is "repaired" with a few strips of duct tape? The repairman knows duct tape won't hold well or for long, but there could easily be 30 minutes or more extra work to do it right. The homeowner probably got bids for the job so time is money and the homeowner doesn't know a good job from a bad one anyhow. It's frustrating, because sometimes it seems like the whole system is set up to ensure shoddy workmanship. The advantages of doing it yourself are obvious.