Sinking fly lines are used for wet line fishing in rivers, and for fishing small lakes, lake edges, and river mouths.
They come in various sink rates from slow to very fast, to suit every condition.
Since most of the trout’s diet is consumed below the surface it just makes sense to have either a sinking fly line on an extra spool or a mini-sink-tip section that can be added to the end of your floating line.
Sinking lines come in a wide variety of sink rates, from a slow intermediate or hover line that drops only a half (0.5) inch per second to a type 7 line than plummets at a rate of 7+ inches per second.
Generally speaking, the deeper you want your fly to fish, the faster sinking line you should use.
As you can expect, the higher the "type" of line, the faster the sink rate. For example, type 4 is faster sinking than a type 3.