This is specifically about using multi materials or colors. It is to reduce the waste cased by the purge tower.
The Prusa Edition of Slic3r has added the ability to purge the extra filament in the line as infill instead of just wasting all of it on a purge tower. Obviously, a purge tower is still needed, but a lot of waste material can be used as infill instead. An option to turn this feature on and off would be needed, in case you are printing with a translucent filament since you could see the other color inside it.