As an independent comic creator, we have an uphill battle to tell our stories. It's hard to find an audience. So we are learning to create one. By posting the whole story for free as I go along, allows my audience to grow. I have learned that they want printed books as well. They and I like have that printed book in hand. It wont be online forever, while in print you can keep it on shelf.
The second reason- is I need to support my family and so this helps. Because people buy the books, it allows to keep working on the story and putting food on the table. A lot of independents work this way, because we have found that it works. If there are webcomics out there that you enjoy- i encourage to go and support their creators if they have printed material.