because it is a great site for artist who want to sell their art in t-shirt form or posters. it would allow for your fanbase to get stuff related to your comic! and I know a lot of artist can get some amount if money off of it!
here is my redbubble
so you can get an example of what it is like!
Before you ask how you can get money out of it, it is based on a percentage, that you can choose! If you sell a t-shirt they have a baseline of 20$ that they take and you add however much you want like. For exemple you can add 7$ more so it would add up to 27$ for a t-shirt!