Typical reasons for back pain are bad posture when lifting heavy objects, sitting for long periods which impacts your spine, or muscle sprain from over-exertion.
If your back muscles are weak, or inflexible, you will lack stamina when performing activities that place a strain on your back, which can cause joint destabilisation, slipped spinal discs, and ligament or muscle damage.
To strengthen this area and encourage the muscles to strengthen and become limber, you should aim to support both the middle back muscles and the spine region.