They do different things. This like, instead of playing an F and a G on the low E string by fretting the first & third fret with your fretting hand, you can "fret" it with your strumming hand, which would free your fretting hand for the other strings around the 12th fret. This, in a way, lets you play a bass line and high melody without moving all over the fretboard wildly. The video on palmfrets.com shows this more clearly.

As a beginner player, this isn't worth worrying about, IMO. It might unlock some interesting composition options for someone writing a song for themselves... Normally, you have a bassist playing those parts for you, though.