That represents the Mobile View / Desktop View switcher. If it’s unclear, then it should probably be a text link instead.
My reasoning was, since the list of categories is very long and not frequently accessed, it’d be nice to make it expand as needed. This gets rid of the bulk of the menu when it’s not needed.
This was also mentioned in codinghorror’s suggestion:
There’s probably a better way to expand the categories instead of a dropdown, but this exact one is already being used and works fine on mobile.