I like platforms like GitHub as it makes it easy to have a public portfolio of your work. Not that there is anything wrong with not having a portfolio but it certainly makes life easier when trying to show clients/employers your skill set.

Pretty much all designers have some kind of public portfolio such as a website with example of their previous work. I see GitHub as a centralised (and, for the most part, respected) place to put your work for developers, even if you don't want to use it for collaboration but simply as an advertising platform.

However it does lead to the same kind of problem you get with employers not looking at a candidate who doesn't have a LinkedIn profile. You will inevitably miss out of some excellent people who don't wish to/can't use such a platform.