The easiest way to explain this, is to use an analogy. Ever have a wood deck? Sealed with whatever? Did any of the decking warp?

Humidity and moisture will warp a stock, over time. You keep it in a moisture controlled environment all year long (to keep the metal from rusting) then you take your weapon out in a wet, humid environment, that wood is going to absorb some moisture, no matter what. How much it absorbs may be minimal, however, in extreme cases, it will warp the wood. Mainly the free float is what is important. The glass bedding in the barrel channel is just waterproofing for the wood (for those that can not seem to remember to clean and dry out the firearms). The bedding is mainly for the action, and recoil lug area.