Your logo is what represents your business in all manner of media, from websites to custom t-shirts. When you create a custom t-shirt with a logo, you are helping to build your brand and establish brand recognition. 75% of brands can be identified by the logo alone.
Your t-shirt logo placement is one of the most important facets of the t-shirt design, just as much as the logo itself. Where you put your logo can determine how much exposure it will get.