I use YouTube frequently, mostly to learn new music. Multiple repetitions get the tune into my head, and then the slow-down feature helps me find the landing notes and then work through any passages that are difficult to figure out. 

(I have largely quit using sheet music, except as a reminder. That might be different if I had reasonable sight-reading skills -- but I don’t. I admire those who can produce real music from a sheet of paper but, alas, that ain't me.)

Later on, when I pretty much know a tune, I sometimes study various interpretations of the tune to discover interesting licks and nuances. I’m just a hack fiddler, so there are many others who can provide good models for me. 

I do not like instructional videos, where a teacher takes you slowly through each phrase. This is excruciating and mind-numbing. I can learn faster and better with my own process.

Instructional videos that focus specifically on technique can be helpful. Some teachers have better explanations and demonstrations than others, and it’s usually easy to find the ones that are most useful for me.

The intrusive ads are terrible. I recognize that YouTube needs to generate revenue, but gratuitously dropping ads into the middle of a performance is just evil -- it destroys continuity and ruins good music. This is greed over art -- they should be ashamed of themselves.

I don’t know if YT has some sort of subscription that would eliminate those ads, but I would consider it.

All in all, YouTube is a useful tool for me. I’m a self-taught and self-directed music student, and YT is an essential resource.

Hope this is useful for somebody. Thanks Rusty for getting me thinking about this.