Oh, the usual... they tell great stories, they bring you to far away adventures, they have characters who mirror the best and worst of ourselves and all-that-literary-jazz. Comics are funny, sometimes satiric. They can be easy, engaging reads even as they show history and not-so-subtle political commentary.
Some comics are so damn good they have illustrations worthy of museums built just for them.
Also, comics make 6-year-olds read for fun. That in itself is awesome. Comics make kids beg parents to go to the bookstore where they can discover other genres and authors. Today Herge. Maybe one day Tolkien?
I was at the bookstore when I overheard a mom sneer at her child, "you should be reading chapter books, not that." That being a Geronimo Stilton comic the poor boy was clutching. As Captain Haddock would say, blistering barnacles! To Sneering Mom, except for the crappy ones, what's wrong with comics? There are crappy books just as there are crappy comics. I know, I am taking her remark too personally, but come on, Danielle Steel novel or Neil Gaiman graphic novel? You choose.