In My Humble Opinion...
I think what Bill Gates, & by extension Microsoft, did for the world of computing can be summed-up by these two videos of Steve Jobs:
Steve Jobs: "We Don't Ship Junk"
Steve Jobs (and Steve Balmer) on Microsoft
Bill Gates (unintentionally) did Apple a massive favour by ripping-off Mac OS to create Windows. If the concept of the GUI had never been applied to cheap, mediocre-to-average quality IBM PCs & their clones, then it's likely that Apple - assuming it was still around today - would be a very small company producing computers with very little practical use.
Why?
Well, while the GUI-based Mac in a world full of CLUI-based PCs would be - in principle - the best computer one could get, it would also have almost no software developed for it.
Microsoft's willingness to put aside "taste", and ship "third rate" "junk" that worked just well enough to make it worth buying - even if not much more pleasurable to use than DOS - made the GUI industry standard, rather than a niche market. This created an enviroment in which the Mac could flourish, even if only as the main competitor product to Windows-based PCs.
And, of course, while Mac OS X may be only the leading competitor in terms of market-share, it is the leader of the market innovation-wise; and so, as Macs gradually become cheaper relative to PCs, their market-share (& so the amount of software developed for the Mac first, if not exclusively) will only increase.
Thus, Microsoft "took the bullet" as it were, supplying the lower-end of the market with a similar product, allowing Apple to not need to compromise & ship stripped-down cheap versions of the Mac to maintain the GUI's viability; and meaning that Apple could focus on quality, not quantity, a philosophy that is fully paying-off some some 30 years down the line, now that production costs have dropped significantly.