I've been posting my comic online for free since I started making it. But I've also been making self published print versions since day one.
Different people like reading it differently. I know people who only read the print versions and get on my case to print the newer issues that are currently only available on the web so they can buy the hard copies and read them.
I also know people who have read the whole comic online for free and still buy the print version because they want to support me and own a piece of the comic.I know I do this for artists I support as well. There's something special about holding the comic in your hand and being able to physically carry it around and show it to people.
As far as professionally getting a webcomic published, it's definitely still a thing that happens even if the whole comic is online for free. I own professionally printed collections of a few webcomics that are these big beautiful (sometimes hardcover) tomes. This usually only happens for the REALLY popular webcomics, but I personally feel like some of them wouldn't have gotten approached by publishers if they hadn't been posting their stuff online and gaining a fanbase.