It is impossibly difficult to break into the entertainment industry these days. Everyone and their mother has a dream of becoming a famous entertainer. Quite possibly the hardest part of the entertainment business is remembering that it is, in fact, a business. Many young musicians and comedians will spend years perfecting their craft only to discover that, at least while starting out, they also need to be sales- and businesspeople. This hits many artists as a huge surprise, and most of them never recover from the blow. In addition to being an expert at your particular art, you must network, market yourself, and book your own gigs.
You, as an amateur entertainer, are creating a brand that is yourself. This brand, like any other, needs to be marketed. Else how will anyone ever know about you? You may never hear about the best guitarist in the world, or the funniest person in the world, simply because they did not know how to market themselves. Now, of course you need to create many things with which to market yourself (website, business cards, etc.), but custom t-shirts are no doubt the perfect place to start.
Businesses use this method to market themselves, and you yourself are a business. Businesses find that advertising themselves on clothing is effective because the clothing travels as far as the person wearing it. Billboards and print mailings have a tried and tested proven record of effectiveness, but these can get costly and can only reach a finite number of people. Not only can a t-shirt reach an impressive number of people in any given day, but t-shirts can be re-worn any number of times. This means that a one-time advertising investment of a few dollars per t-shirt can reach thousands of people, and continue to do so for as long as you like.