Merch stores are designed to achieve two goals. Revenue generation and brand awareness for the creator or business.
The revenue generation point is obvious. By selling print-on-demand goods the creator can provide a physical item to a highly engaged fan. Theoretically, both parties win here - the creator gets some revenue and the fan gets a cool product and the good feels of supporting a creator they like.
The creator gets a secondary bonus with additional brand recognition. People who buy (and wear) merch from a creator's shop are evangelists for the creator. They will drive more views, fans, and ultimately sales.
At JohnDav I've created a small selection of print-on-demand items as a testbed. This is a learning experience for me and I will make mistakes, but if you're a fan of what I'm doing then by all means check out my store.