Buying a position in a company like Samsung (solid-state memory or screen tech), Foxconn (manufacturing), LG Display (screen tech) makes sense. Apple is a hardware company, so pretty much all their investments are in hardware or software that increases the value of or margins on their hardware (like chips designers and firms that increase the value of OS X or iOS, which in turn increases the value of Mac and iOS based devices)

IMHO Foxconn makes more sense than any other investment because Apple is directly contributing or jointly inventing cutting edge manufacturing technology with Foxconn. An example of such technologies is the precision cut aluminum enclosures of the aluminum Macbooks.

An investment in Foxconn would suggest that Apple has a desire for being the hardware basis for a lot more devices in a person's life than just a computer or smartphone device. Since the future is an internet of things, the best way to get a foothold in the premium end of the internet of things is to have a solid foothold in the largest and most capable manufacturer of electronic things.

On the other hand, Foxconn has a $1.1 trillion market cap, so they could at most purchase a bit less than 10%.

A sizable position in Foxconn could also virtually guarantee that Apple is the only company that will have access to manufacturing technology and quality in volume that is always 1-2 generations ahead of its peers. You can't compete with Apple on quality if you can't get access to the hardware manufacturing capabilities until after Apple abandons it and moves onto newer better technologies. It's the equivalent of a hardware checkmate.