It's done to monitor aircraft performance for a variety of reasons. It can be an early way to detect an abnormally high fuel burn which can be critical on longer flights. It can detect a fuel leak. The FAA and maybe some other agencies allow "redispatch" flight plans. On these, a minimum fuel value at a certain point on the flight plan must be met or exceeded, otherwise the flight has to divert. It's important to detect and solve the problem early if it appears that you might not make your redispatch fuel, otherwise you may find yourself diverting.