There are lots of reasons, in my case! Part of it is because I've spent my entire LIFE reading (and having been read to from) comics in print-format, and a lot of the webcomics I've read - and I've been reading webcomics for 16 years - have also been in print-format. It's simply something I'm used to.
Part of it is because I find it easier to plan my comic in page-format. A page is a unit that most of us are used to dealing with, and it's easier for me to plan and control the pacing.
And part of it is, yes, to have it printed at some point. I've already collected chapters 1-3 of Grassblades for print, and it turned out lovely. I've always loved printed books - both comics and prose - and I love having stuff in my bookshelf that I can pick up and flip through whenever I want. Maybe it's because I've always lived in a place where power-outages are frequent and the internet connection is poor, but I've always preferred books to digital mediums, because books don't require a functioning internet connection, and they never run out of batteries.
(honestly, it might also be because my grandma was a librarian, and books have been omnipresent in my life).
Then there's the fact that no matter how much work and effort I put into making a digital thing, and no matter how many people end up reading and commenting, it somehow doesn't feel as real as having a printed book in my hands. There's a heft to a printed book that is kind of lovely, and it's a tactile thing, too. Comic-pages sure are nice on a desktop monitor, but it just can't quite beat 130 gram silk paper, you know?
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Also, when you get right down to it, if you want to sell your comic, physical versions are easier to sell. We're in that weird gap in a shifting economy where digital means of payment (via Patreon, Kickstarter, Gumroad, etc.) are available, but we've spent so long in the pirating waters and the free-stuff-for-free mentality that people are kind of reluctant to pay for digital stuff, I guess? But people have NO problem paying for a physical book. They understand that it cost you money to create, because it's physically there and they can touch it.