Every owners manual I have read specifically says to NOT use any supports under the slides. The problem is if the RV settles one one side, it can cause the slide to be in a bind and cause mechanical damage as well as leaks.
The reason stores sell them is that some folks never read the owners manual.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot