This short film illustrated the depth and retrieve control an angler has by utilizing different sink rates of fly line. In this film the slow sinking "hover" line is put against an intermediate.
Hover and clear intermediate lines are excellent fish forage lines. The hover sinks at a paltry one inch per second. The clear intermediate sinks at a rate of one-and-a-half to two inches per second. These lines provide an exceptional horizontal presentation and work well in windy conditions.
Line choice, whether hover or intermediate, is a function of water depth and retrieve speed. When fish are active, you can use a quicker retrieve. In these situations the faster sinking, clear intermediate is often the best choice.