This would be useful in many different scenarios. Here is one scenario:
Player 1 makes a thread asking what you need to make a bucket. Player 2 replies to the thread Player 1 made. In that reply, Player 2 explains to Player 1 that a bucket is made with boards and rope. Player 1 realizes that the post Player 2 made could be helpful to other players, and so he marks it as the solution, which other players will see in the original post.