Tubeless tires have an additional layer of butyl rubber on the inside to hold air. Tube-type tires do not, and may be somewhat porous as a result. This is what Krpntr was getting at.
There was a guy on a GS on his way to the MOA rally three years ago who bought a new tire locally. It was a tube-type tire that was installed without a tube by the shop. He made it about 30 miles north before it gave up and was rescued by my local Airhead mechanic. Just a constant slow leak.