A ball-end hex L-key will allow you to get into otherwise impossible areas to tighten up a screw.  The ball shape on the long end of the key allows for a misalignment of up to 20 degrees. The short end of the key is square-cut to provide maximum surface area when you have a clear shot at the screw and want to use the leverage of the key.  
The 2.5mm drive size is found on M5 set screws, M4 low-head screws and M4 button head screws.
 The 3mm drive size is found on M4 socket head screws.