The most obvious, a map, another option could bee a quick inventory instead of opening the menu thousands of times and wasting lots of hours just on the menu.

I was really thinking that it should have been used for a map!

It was going to be the Sheikah Slate, but then Nintendo wanted the game to be a launch title for the Switch. And they didn't want the Wii U version to be slightly better, so they moved the Slate to the minus button instead.

Switch was the deathbed for the Wii U. :(