Other than its most basic function as clothes, custom T-shirt can actually do a lot of things for us.
Firstly, it can help us outwardly express interest in a specific thing: movie, TV series, comic book or a character featured in any form of art; a music band, singer, actor etc. This way it is able serve as uniform for a fandom, fanclub, meeting of a community.
It can also be used for more abstract functions such as delivering a message within a community, showing our support for a cause, raising awareness on a specific matter.
We can totally turn custom T-shirts into a business. Idea and method are what we need to make profit of this kind of special goods.
Ultimately, custom T-shirts enable us to express ourselves to the world, let us speak our minds freely and creatively which is a very positive thing.