A lumbar support is an important accessory to help maintain proper posture when sitting and so avoid low back pain. Many individuals fail to maintain proper posture when seated, particularly after extended periods. We tend to let our shoulders and upper back hunch forward instead of keeping the back and shoulders straight. The result is excessive pressure on the lower back from the weight of the head and upper body. That in turn can result in lower back pain after sitting for a while.
The purpose of the lumbar support is to help us maintain a space between the lower back and the back of the chair when sitting. That in turn forces us to keep the upper back and shoulders straight, maintaining proper lordosis. The resultant outward curve in the lower back reduces the pressure of the weight of the upper body and, with it, low back pain.
Lumbar supports can come as part of a lower back brace or can be bought separately.