Tiny End Mills

I always wondered  for purpose what these skinny, fragile-looking end mills exist.  I figured it out.  I want to cut a square slot for a telescope ring.  You really cannot cut a square bottom slot that is square in the corners with a large end mill.  But I am using a .125" end mill to cut a .22" deep, 1.6" wide slot.  It is not really square in the corners, but close enough.