Battery grips are multi-functional accessories for DSLR and Mirrorless cameras. Their main use is to provide more on-board battery capacity so photographers and videographers can shoot for longer without running out of power. They also act as a larger camera grip, making it easier for photographers to firmly hold their cameras when shooting in either landscape or portrait orientations.
Battery grips overall add more control for handheld photography, and some units even have an integrated shutter button on them. So, when the camera is thrown into a vertical portrait orientation there’s a shutter button available at a more convenient location than the camera’s standard one.
The weighty and robust design of most battery grips can also give a DSLR or Mirrorless Camera a more balanced feel. Many photographers who start using battery grips for their handheld work quickly appreciate the balance and the flexibility it gives them to swap their camera from hand to hand, as well as to rotate effortlessly (and confidently) between landscape and portrait orientations.