Why do you have to keep your seat belt fastened after landing?
Ground operations are just as dangerous as air operations. Strictly speaking the deadliest aviation incident actually happened on the ground (not saying seatbelt played a role or not however severe accidents do happen on the ground). The comments cover things well, (keeping people seated, keeping people from accessing over head compartments etc.) Planes are not safe simply because they are on the ground, they are only safe from flight related incidents and once on the ground they are susceptible to ground related incidents. By some estimates 27,000 ground incidents happen a year. The fact remains there are plenty of things to hit on the ground and plenty that can go wrong in taxi. These accidents are classified by ICAO under (RAMP) and (GCOL) codes (and potentially others). Boeing has a breakdown of accidents by flight category (taxi and ground ops make up about 10%).

Furthermore here in the US under the FAA FAR 91.107 as well as FAR 121.311 in the commercial case(basically the same wording) requires it


  ...(3) Except as provided in this paragraph, each person on board a
  U.S.-registered civil aircraft (except a free balloon that
  incorporates a basket or gondola or an airship type certificated
  before November 2, 1987) must occupy an approved seat or berth with a
  safety belt and, if installed, shoulder harness, properly secured
  about him or her during movement on the surface, takeoff, and landing.
  For seaplane and float equipped rotorcraft operations during movement
  on the surface, the person pushing off the seaplane or rotorcraft from
  the dock and the person mooring the seaplane or rotorcraft at the dock
  are excepted from the preceding seating and safety belt requirements....