There is a reason, it’s not a practical one… (in my opinion) but it is a reason. You will in fact be judged by whether or not you use or own a Mac as a Graphic Designer. In reality these “marketing optics” do matter to some people, even if they shouldn’t. Many designers feel that if they can’t afford a Mac they won’t be taken seriously, so they’ll even invest in buying a used Mac that cost twice as much as a newer Windows PC. Many designers find themselves turned down for jobs if they prefer using a PC rather than a Mac. It’s astounding to me that in 2015 the question of which operating system you should use is still a debate.