Simple, the better a dry fly floats the more chances for strikes. Attach air to a hook and the fly will float better. Combine air with traditional materials (feathers and fur) add foam and the hook becomes a super fly. This is the rational behind using polyolefin heat shrink tubing to tie dry flies. Below are some of the methods that can be used to attach air to a hook.
Sample Flies
If you would like more information just type in “Tying Unsinkable Dry Flies” in your search engine.