[rickst29]

They are put there so that you can brake the Trailer independet of the TV brakes for example, in case it is skidding the the side. In such a case, it is best to brake the Trailer much harder than the TV, by using the lever. The TV stays a bit faster, and pulls the "dragging" Trailer back in line. So if you get a good one (the Tekonsha "Prodigy" is very popular among TM owners; I love it), the you can and should test that your Trailer brakes are working - every time you begin towing.

First Test: push the lever, and feeling the the Trailer resists movement of the TV quite strongly. Then (2nd test) Release the lever and start moving slowly (5 MPH); stop, and verify that the Trailer creates "Drag" on the TV while you are stopping. As you begin towing, test again (3d test) at roughly 20 MPH. Feel the Trailer pulling on the TV, a bit, when you apply the brakes with moderately light force, as you slow to a stop.

You always want the Trailer braking to be at least as "strong" as the TV braking -- otherwise, the Trailer tries to push the TV whenever you apply the brakes. (And that's something you absolutely don't want, the Trailer trying to control the TV.)
You should also, I think, search for Threads about "setting the prodigy controller", and read them carefully.
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I feel that you should be sure to buy a more expensive "proportional" brake controller, such as the Prodigy, and take some time to set it up. The cheaper "time delay" controllers cause more wear and tear on the brakes, and the Time Delay can be catastrophic if you need to apply the brakes HARD AND IMMEDIATELY, in an emergency.