Using a battery grip on your DSLR solves two problems. First, it doubles your shooting time by doubling your battery power and second; it allows you to turn your camera 90 degrees for portrait shots without the awkwardness of reaching around for the shutter button.
A DSLR battery grip also introduces potential problems. First, it adds additional weight to your camera, making a potential heavy camera rig heavier. Secondly, it makes your camera appear massive which can further intimidate camera shy subjects.