Traditionally - the Mac was a superior computer to a PC. Back in 2006 and before most PCs were mainly office PCs - and high-end PCs were something of unheard of.
Even gaming didn't really need a superior machine to run games, but they were a bit more expensive than a normal PC - and a lot cheaper than a Mac.
Macs were vastly superior computers back in the day.
But nowadays - PCs are just as good - you can buy a high end PC that's x2 as good as a Mac and 1/2 the price.
Why do designers use macs? Legacy I suppose. But nowadays it's much more than that - it's the genre of "it was here when I got here".
Quite like a lot of preflight profiles out there, they were built by someone who knew what they were doing - now there's a mac monkey in their place and they just see red flags, and no idea how to fix them.
I design on a PC laptop at home, and I work on Macs in work.
There's litereally no difference in the software from Mac to PC - except keyboard shortcuts.