Why are "Graphical abstracts" advantageous?
I think it may be a substantial advantage. While I've never had the chance to create one, I really like them myself for a number of reasons. 


When searching through volumes of literature, they are much easier for a reader to take in at a glance than a traditional abstract.
They often provide substantially more information about methods and results than can be packed into a traditional abstract.
They are ideal for sharing on social media such as twitter, making it easy for you or others to promote your work.


And after all, a picture is worth 1000 words. Which is more than one gets in some glossy-paper journals these days.