1- The tooth was not fully erupted (grown in) when the first brace was put on and now the position has to be changed to align the tooth in question with the other teeth.
2- The tooth was so out of position at the beginning that the bracket could not be placed at the correct spot on the tooth to begin with and after some movement, the tooth has been corrected to the point where it can be repositioned properly.
3- For purposes of detaining and finishing where "tweaking" a tooth's position with the wire would be more cumbersome than just repositioning the bracket.
4- An opposing tooth is biting into the bracket and the bracket position is causing damage to the opposing tooth.
There are a few other reasons but these are the most common and even these occur rarely.