Do you learn arpeggios and scale just to play them?
Are the good for fill ins?

Yes, but the important part is neither. Practicing both the Arps and scales gives you a command of the instrument – like lifting weight helps whatever.

I was a fair guitar player, hmmm, maybe just above fair. (smile)
When my guitar teacher gave me arpeggios and scales to practice, I was all over the guitar – comfortable in any position.

The speed of a note in my head to the note on the guitar was many times faster. In short I became a better guitar player.

When I went to a keyboard, I used the same principles; my learning time was cut in half.

Just saying, John C