There were a lot of reasons. One reason was that it made Gates feel good to help his old friend. One reason is that Apple and Microsoft did a patent deal at the same time. One reason is that Apple agreed to make Internet Explorer the default browser on Mac OS. One reason was that it might have decreased anti-trust pressure on Microsoft, though it didn’t (the judge said Apple and Linux didn’t have enough market share to matter).
One of the biggest reasons was that Mac users were extremely profitable. Mac users bought millions of retail copies of Windows, and they bought millions of copies of Microsoft Office either for Windows or for the Mac. (Mac users paid far more for Windows than PC users, who got it pre-installed, and they paid slightly more for Office. Mac users were therefore worth more to Microsoft than Windows users, on average, though there were not as many of them.)
When the Mac was launched in 1984, Bill Gates appeared on stage with Jobs, and the graphical versions of Word, Excel and PowerPoint were all developed first for the Mac. Microsoft Office also appeared first for the Mac. (The Mac didn’t have as much software as Windows, so there was less competition, and Mac owners paid higher prices for software: it was good business.)
Apple fanboys love to hate Microsoft, of course, but Microsoft was always the Mac’s biggest supporter, and Gates offered to help Apple to make Mac OS a global standard. Having Microsoft Office gave the Mac a huge advantage against Amiga, Atari ST, Acorn Archimedes and other rivals that didn’t.
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