Smart Troll is is the only system that enables you to use multiple probes that can be attached directly to your fishing line to send depth and temperature data from the lyre to a display on your boat. Without Smart Troll the depth of your lures and other gear is an educated guess at best. Some captains can get pretty close some of the time but there are so many variables it that are frequently changing it is a very difficult task to precisely locate your gear without Smart Troll.
Many captains who use Smart Troll primarily target water temperature. For example, King Salmon preferred temperature is near the mid 40 degree range. With Smart Troll Captain's put their lure at that temperature knowing that no matter where the King Salmon are at the moment they will return to their preferred temperature sooner or later.
You can also use the optional paddlewheel attachment to attach your probes to a downrigger cable a foot or so above the ball. Smart Troll is the only system where you can have probes on multiple downriggers at the same time. On multiple lines at the same time or some probes on the downrigger and some probes directly on the fishing line.