Useful if you like to evenly overbuild your planets. It works only if you have enabled 'Advanced game mode'.
It adds an input box at the bottom of the planets page where you can enter a percentage. If you click the button all overbuilt planets are checked that are below that given percentage.
So if you have 200 planets for example that vary in overbuilding level between 250% and 124% and you enter 180, you can quickly select to build only on planets below 180%.