Why should you be in proper gear before going down a hill in trailer tractor?

Well, if you have to ask this, maybe you should rethink your profession.
Tractor-trailers are heavy... weight + force of gravity = a lot of force rolling down a hill. You get into the proper gear so that you're able to maintain control of your vehicle, and do so without burning out your brakes. Trying to downshift on a decline is a very troublesome and dangerous thing to do, and, if you miss a gear on a 6+% grade, you're not very likely to be able to recover, which ultimately leads to you not being able to control your descent down the hill, or, if you do manage to get it back into gear, you're probably going to end up doing it in a manner which destroys your transmission.
People who own trucks generally don't like them getting wrecked, or having to spend money on repairs which wouldn't have occurred with someone who knew what they were doing behind the wheel.