Actually, there are a few reasons for using that plug-in.

One reason is with up-to-date browsers, YouTube decides what files you can get ... for your best experience, of course. :\ If you're on a slow line, you can't force a higher quality video (with all that stutter? tsk, tsk). It's just not on the list from which you would pick!
That's one giant leap (backward) for user freedom, more small steps (forward) for comrade YouTube.
:O

Another reason is that CYS (Complete Youtube Saver) lists all YouTube video formats. Here's the overview list (there is also a details list, with codec types, file-sizes, the works, which you select what to show) ..

And, as you can see towards the bottom of that list, there is built-in audio conversion and saving in a variety of formats (all using FFmpeg). What could be easier and still thumb your nose at Google/YouTube statist ambitions?

And if you act within the next 10 minutes, we'll throw in a 2nd one ... absolutely free! Just pay shipping and handling!