In regards to external services (Dribbble, Behance, and Deviant) what is the purpose of having a portfolio website but you link all your images externally and take a viewer away from the main site and purpose?
From a user experience perspective it doesn’t make sense to do this. Generally users wouldn’t appreciate a link to a site that has the same content.
I can think of cases where it would make sense, however. An employer or possible client could view it as you being very engaged in the graphic design community. Showing you are active at other sites conveys your passionate about your craft. Even if a user doesn’t click those image links it immediately conveys this through recognition.
Secondly, if these sites were used to show other things in your portfolio, like a design process or contribution to another project, I think it would be beneficial as well. One of the really cool things I’ve come to learn about Behance is that designers will sometimes show the steps that were involved in creating a design. It demonstrates their expertise.
There is a final reason that isn’t a good reason, but I am pretty sure is the main reason for this choice. Other graphic designers are doing it. Brands engage in similar ways with social media. They don’t do it because they have determined the efficacy of external links to be great, they are simply followers who feel they will be left behind if they don’t. Following the crowd, however, has never been the best way to distinguish yourself.