Answer:
if your selling shirts, everyone wants a cool new design and not just a plain shirt. everything now-a-days is fashion and if isnt the trend or a new trend no body will wear it.
NOW IF YOU TALKING ABOUT A RANDOM PRODUCT YOUR TRYING TO SELL, Advertisement is a huge deal. advertisement can be shown in different ways such as billboards, SHIRTS WITH PICTURES OF THE PRODUCT, And etc....
it might catch someone attention and ask the person with the shirt on about who made it how can i get one and stuff like that you're basically promoting the product to gain popularity so the product can get more customers which also mean more money
