What exactly is the point of this product?

I need to put up chair rail, and it seems like having something super sticky - stickier than my normal thinset - would be a good idea. So after a visit to my local Home Despot, I picked up a little tub of Ceramic Tile Adhesive. Very sticky stuff.

After I got it home, I took a closer look at the disclaimer in fine print. It says "Not for use in areas of prolonged water exposure". Now I ask you, what does one usually tile? Oh yes, backSPLASHes and TUB surrounds and SHOWER enclosures. What do those things have in common? Let me think. Oh yes, PROLONGED WATER EXPOSURE.

So what EXACTLY does one tile using ceramic tile adhesive where there ISN'T 'prolonged water exposure?