This is a feature to allow exporting the active (selected) UV map instead of exporting all UV maps in the file. Some software (like Substance Painter) at the moment only works with a single UV map and if you had several UV maps you wanted to use you would need to create a copy of the mesh, remove one UV map, make another copy of the original mesh, remove the other UV map and so on. This gets worse the more UV maps you have.
This patch attempts to solve this by adding an option (boolean) for enabling the feature of exporting the active UV map. That way you can keep your original mesh and just export it twice (after changing the active UV map) and this way it's a non destructive action, or at least saves you the trouble of copying your mesh a bunch of times and removing the UV maps.
This was made to improve the workflow for exporting a mesh with a single specified UV map for texture creation purposes, but could also be used for any other time you need to export a mesh with a single specific UV map.