Imagine our hero is crossing an enchanted room, so he starts to float due a powerful spell. In this case, while the hero is within this room, gravity should be contrasted by a (magical) force. As a result, we need to specify that in that room, there is a force that makes our hero levitate.
Similarly, imagine the hero drops a box in a lake. The box doesn't sink at the same velocity as it falls; moreover, it might float. Therefore, in our game, we need to specify that the region delimited by the lake has special physical properties.