A line lock locks the front brakes so you can do a burn out without heating up the back brakes.
A trans brake, which there is several different versions on the market, essentially locks reverse and 2nd gear together so that you can push the pedal to the floor and build 15 lbs of boost and not push the car through the staging lights, which happens quite often when trying to build boost by just using your foot brake. It lets you launch really hard!! Only if you have slicks though.
Both of these are for getting a good launch at the track.