Barre chords have a few uses:
1. If you are playing with another guitarist, and they are playing open chords, you can play those same named chords as barre chords in a different position / inversion higher on the neck to change up the sound. That way, you're not both playing exactly the same thing. It adds some extra color and variety. Anders covers this in his Acoustic course.
2. Barre chords can help you play in different keys quickly, since your index finger is working as a capo. If you're playing a song with G and C barre chords in 3rd position (let's assume key of G), and you want to move everything up to key of A (A and D chords), you just play the same shapes at the 5th fret. If you tried to do the same thing with open chords it would be all different chord shapes. That's why learning those basic barre chord shapes on 6th and 5th string are so handy, especially for rock music...
3. Barring also simply gives you bigger chords, since you can hold down more strings at once. 6 strings and only 4 fingers...
Hope that helps!
Carl King[br]GuitarTricks Video Director / Producer