Earlier quoted context omitted.
It is for usability. Ubuntu's only weakness is that at the pixel level it's a bit uglier than OSX in some areas... and that by default it doesn't come with quite the same level of fonts...
Fonts are in the eye (or rather, visual cortex) of the beholder. I got used to the Mac fonts, but at first I thought it was some kind of bad joke or misconfiguration. They seemed annoyingly blurry. My theory is that one's brain gets used to the particular fonts and may not like change very much.
This could just be my own aesthetic taste, but I think it could also be due to those companies having highly paid font designers working on those details.