MS originally offered Office for Mac and agreed to continue doing so as part of the deal was they gave a cash injection (and gained shares in Apple) to keep Apple afloat at a time when apple were in very real danger of going under for good. Bill Gates/MS knew to keep the monopoly vultures from landing they had to keep Apple as a viable competitor, hence investment in them. Software sales are software sales, Apple's computer sales compared to the number of PCs sold is tiny, but just enough to keep the monopoly vultures from landing still. The same kinda thing really could be said to exist between AMD and Intel with Intel allowing AMD to have an x86 licence. Intel may even invest in AMD to prevent them going bancrupt and getting those dreaded vultures landing on them. MS are diversifying in general with regards to software, they're contributing to Open Source projects a fair bit these days, as it helps them in some respects as they increasingly use some of that Open Source software now too.