That's a secondary use. I could be very very wrong but I always thought that the knub was for your thumb so it doesn't slip haha. Either way as long as it works for ya!!!

Now this is some mildly interesting content

Excited British noises

If it's a bigger mug i'd say it's for the thumb since the teabag string can be wrapped around the handle as well.

I always used it as a thumb grip, but damn, this makes it ever more useful

Mildly ceramic balloon knot

You could be right. I find the knob is ergominally terrible as a thumb pad. Teabag strings aren't long enough to wrap around the handle on this mug so the knob is very useful for that.

That's quite possible. I find doesn't really work for thumbs and it does work well for strings though.

EDIT: Spolling

Just saw the short string, i only knew teabags with a string that's at least double the lenght.

That's the same length string as any other teabag I've seen.

Always nice to have a handy knob. :)

There ya go then