If you click a YouTube link online or in another app, it automatically opens in the YouTube app.Who uses the YouTube app to view vids anyways? I think YouTube mobile is far better. I can even watch most YouTube vids native on the full desktop site since most vids are converted nowadays.
If you disable the YouTube app to stop this from happening, it stops embedded videos working on a lot of sites.
I also like to use the app because it always stays signed into my account, whereas I have to keep signing in if I'm using Safari.
And the website doesn't support features like playlists, continuous playback etc. Not that the app does either, but it seems like it would be much easier for an app to implement than the site.