It can be fun and exciting to wear a tee shirt that says something cool in particular. Some people like to throw them on on occasion, because it will draw some attention in public.
Another reason why people will go with funny graphic tees is that they want to support someone who is behind the design. Maybe it is a band that they are supporting, or a particular artist who put together the piece.
They are not always appropriate for any type of situation, but it’s nice to wear a graphic tee around the home if possible. Some people don’t think too much of it, and are willing to make adjustments in their life to have a few more casual days from a clothing perspective. It is very relaxing to wear something a bit more laid back, rather than dressing up all the time.