The function of a battery grip is in the name itself. It’s an accessory that plugs into the bottom of your camera, providing room for an additional battery — and an extra hunk of plastic for the photographer to grip. Some grips are even called portrait grips, because they include a second shutter button that’s more easily accessible while taking shots in portrait orientation.
Your average DSLR is ergonomically designed for comfortable landscape shooting — not portrait. Grips exist to make portrait shooting more comfortable by including a second shutter button on the side of the grip. When the camera is rotated, you can use that button to take portrait photos as you would a landscape shot.