Mystery filament cleaning fluid Whilst looking to fix a different problem I had I came across this video.

It's a filament reel holder, but at 02:40 the person uses a mysterious fluid.  I haven't seen anything like this before (though had wondered).

Mystery cleaning fluid.

What practical problems would this cleaning fluid mitigate?
That's a filament cleaner/oiler combo. The black bottle is a variety of machine oil. 

The effect of oilers on prints is heavily disputed in the community. The suggestion of plant oils is a very bad one as they can create residue that stays in the hotend and create clogs.