Minecraft wiki entry for Update_LWJGL_(Legacy), associated text: Updating will fix some input bugs where keys seem to get stuck if you click at the same time. It also removes the "ghost" mouse pointer clicking.
Some users report a performance increase upon updating.
This can also potentially fix some issues regarding sound not working/cutting out.
It has been reported to fix intermittent crashes on Windows.
It has also been known to resolve the "cannot get JAWT interface" error for Mac users using Java 7.
Some Linux distributions, such as openSUSE, require LWJGL to be updated before Minecraft can even run. Trying to run Minecraft without updating LWJGL may create a black screen upon loading. If you cannot play Minecraft on your Linux distribution, chances are that this is the problem.