I actually asked myself this, but for my own comic - "why are you trying to print this if you're not trying to make money?" In my case, it's a combination of wanting something tangible to hold in my hands, wanting the experience of flipping through the pages with an actual book to see if I captured the correct feel, and just feeling more "accomplished" if something I've made existed in "real life".
As a buyer, I purchased the 3-Panel Soul books earlier, both to support the creators and because I loved their art style and wanted it on quick reference, without having to be online and read through their entire site.