What advantages do Macs offer designers?
From a functional standpoint, I'm not aware of any differences in the most commonly used software at all (Photoshop, etc). The only difference might be if you happen to prefer Mac OS vs Windows OS. Macs do come with some software such as iPhoto that may be seen as an advantage compared to a Windows PC. In regards to your question, this is probably irrelevant because serious design work is typically not done using the software that comes bundled with the OS.

Another reason that Macs could be preferred is that they tend to have better color reproduction than many Windows machines, particularly in laptops. That's not to say that they are necessarily better than Windows computers, just that Apple typically uses higher quality screens than Windows PC makers do and so for design that would definitely be an advantage. With a desktop this isn't as much of an issue because you can use whatever monitor you want, but with a laptop on-the-go you're obviously stuck with the screen it comes with. Here are some comparisons showing black/white levels, contrast, color accuracy and gamut of several Macbook models vs Windows laptops: http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/5

I am both a Windows and Mac user and I find that I like both systems for different reasons. I have used Photoshop and Lightroom on both systems and have no preference either way.

Bottom line, there's nothing inherently better about Macs compared to Windows. Just differences that may cause you to like one system over the other. You pick what's best for you.

For some additional reading here are some points from both side of the fence arguing for PC vs Mac in design work: http://designfiles.net/blog/mac-pc-graphic-designers/