For humility, politeness and respect
In Japan, bowing to a person is an expression of humility and politeness. Therefore, the lower you bow, the more humble you seem and the more respect you show to the person you are bowing to. If the person is of a more respected status, you should definitely bow lower than the person.
When bowing normally as a greeting, you do not have to bow that low. You are doing that to show your respect and to be polite.