So this question is basically the greatest struggle that ALL content developers online deal with, from webcomics to youtube channels to Netflix.
There are many methods and options to try and I'm not going to say any one method is the correct and only way to do it. However, how I see it, posting my comic online is a way to advertise it, build a reputation, build a readership, and build revenue streams. And the best way to do that is to put my best stuff out there and get as many eyes on it as I can. The content I post isn't exactly free since I earn ad revenue off of it, so in a way, a reader pays for it by viewing ads on the website. [They also have the option of buying printed books and downloadable pdfs off my site] Also, I have a few dedicated patrons that fund my series on Patreon. Building revenue streams is an online content creator's friend.
Keeping material behind pay walls generally don't seem to work for webcomics unless they are XXX rated. Readers would truly have to be very into a series inorder to pay a subscription, however, a lot of people genuinely prefer reading printed copies so selling books along side a free posted webcomic isn't hurting from sales all that much.
Also, as far as conventions go, webcomics is still not a household term. So many people still have no idea what webcomics are. So going to conventions with printed book is fantastic and you can actually get people interested in your web series that way as well. Then they get hooked to your online series, viewing it online build ad revenue, and they purchase the books as you release them to complete their collection. Or flip side, the convention goers found your series online first and excitedly met you at conventions to buy signed copies of your books [always fantastic when this happens to us].
Think about youtube creators really. They post hundreds of hours of seemingly free content on youtube yet they're earning money... How? Ad revenue and patreon. The same tools us comic makers have. If you develop your readership [which takes time] it's possible to make a living making webcomics through ad rev, patreon, and printed book sales.
People put more value in the things they buy, it's true. So having an option to buy books and merch along side of a webcomic series, I highly recommend.
One last thing to consider, if you're ever interested in courting publishers one day with your series or joining highly regarded collectives, these companies will take your work more seriously the larger your readership is. If you can prove to them that you have a product that people want, you're less of a risk to sign on and more likely to land good deals with them. Then those publishers will take care of printing books, getting them into stores, getting you to conventions signing books, and that's when you're really start seeing bigger returns from all your hardwork.