Answers some readers have given me:
There are people who just prefer reading physical books -- the feeling of turning a page is different from the feeling of clicking the "next" button.
At conventions this comes up in a slightly different way -- buying a book is gonna be more convenient than saving a business card and remembering to look up the URL when you get home in order to read it for free. Some folks go "oh, it's online, cool I'll read it there!" -- others go "Wellllllllllll...nope this sounds neat, I gotta go ahead and buy it now."
Some have said they like having books to re-read the comic at any time without needing internet access!
Some people genuinely just want to support the artist.
Physical books are also nice to gift or loan to friends -- I've had folks buy books with the express purpose of showing the comic to a friend that wasn't into webcomics, but would probably read a comic book. I've also had a couple parents buy books for kids that read the comic, or friends buy the books for their friends who were big fans.
For me, I don't tend to buy unless there's extras -- even if those extras are just "a little exclusive 3-page comic and a nicely drawn map" -- but for comics I really really love (like cucumber quest) it's a cool way to show tangible support to an artist that feels significant, and it feels like a kind of fan-cred to have all the books on your shelf. At conventions, I've bought books for comics that I loved but had a hard time following and felt that I needed the book in front of me to read it all at once, and I've also bought physical books from artists that I admired and wanted to walk up to the table and show my support for!