Why teeth whitening?
To start, you might want a more appealing smile in general, but are not prepared/ready to go with a major treatment plan. Whitening will give your smile a nicer spark and that is enough for many of us.
The reason why your teeth have discoloured can vary widely. That could go from a natural discolouration (which is a normal effect of time) or staining caused by foods and drinks. It's also possible the discolouration is caused by damage to your nerve and dentin (the inner side of the tooth). This intrinsic discolouration might be harder to treat with whitening treatments.
Alternatively, you might need teeth whitening if you are considering bigger cosmetic or functional treatments such as crowns, veneers, implants or bridges. You could consider teeth whitening first to lift the shade of the natural teeth in order to match the prosthetic lab work that will be fitted.