1) The presence of the foam line means that is where the majority of the current is flowing. When fly fishing you have to realize that trout see current flow as a buffet table. That is where their food comes from as the current is what washes the food downstream to the waiting fish. This is where the spent insects are concentrated and free flowing, so why not lay your offering out into the foam as well? It works after all, at least in my experience.
2) The foam line is also a good indicator of where the actual seam is located. The seam is where the fast water and slower water meet. This is one of the best places to target actively feeding trout. They can hold in the slower water, which is naturally much easier for them to do, and still see the food that gets washed down in the faster current. When they locate a possible meal they can dart out into the faster water, grab it, then return to the softer water to wait on the next offering to come by.
3) The emerging insects can also get caught up in the foam line as they try to break through the surface tension of the water in order to hatch. This is a great time to swing your soft hackle fly through the foam. In fact, many times the trout are actually feeding on the emergers versus the adult flies when spotted feeding in the foam. The "bulge" displayed as they feed on emergers just under the surface is often mistaken for a rise. This can be a critical mistake for an angler, as they will usually concentrate on the emerging pattern instead of the adult imitation. You can literally try every dry fly in your box without any luck. It can be that technical at times, especially when trying to fool wild, native trout.
These are a few of the top reasons that it pays to fish the foam lines, especially when you're on new water that you are not familiar with. There's no need to know where the drop offs, ledges and other holding water is located when you can easily spot the foam lines and concentrate on those. Foam lines are a secret that the locals just can't hide. Give them a try and you'll see why we wish that we could!