As someone who used to own several anime T-shirts, I can see the appeal of clothing yourself in images of your favorite series. Some might argue it’s silly to turn yourself into a walking billboard for any product or organization, but if the art is part of what attracted you to the show, wanting to wear a piece of it doesn’t necessarily brand you as some sort of mindless slave to consumerism.
If nothing else, it’s a way to communicate your passion for your hobby to those around you, and can occasionally serve as an ice breaker for meeting like-minded individuals. In a sense, anime clothing is the uniform of an anime fan.
Although, in the case of this incredibly dedicated otaku, it’s more like his armor.