One use would be as an electronic kneeboard to display maps and other stuff.
What a tablet could offer your cockpit is a touchscreen monitor that can be used to diaplay instruments or things like touchbuddy.
The problem would be that the tablet is a seperate computer so you need some way to connect the sim PC and the tablet and have software that can use this connection.
IIRC there is software that will extend a desktop and display the extra part on a second computer. (but I don't recall it's name

) In that case all software can run on the sim PC.