I've seen several people talk about it, so let me explain it to you.
Ok, let's going to cut to the chase.
1. Why are the chapters blocked?
Webnovel is linked to Qidian/Yuewen. The place where the novels you read here is published. It therefore officially supports authors whose content you read for free on pirate sites until now.
The chapters are blocked to be fair for the Chinese community. After the first 200k words, they have to pay to read on. It is, of course, right that this should also be our case.
Which brings us to our second point.
2. Why is it not free anymore after a while?
Well, it's simple. Because the novel you were reading reached that 200k word limit at this chapter.
Now. Be aware that the money you use here will go directly into the pockets of the author whose content you are unlocking and his translator. But mainly to the author (and Qidian/Webnovel of course).
So, you can really support your author by buying the chapters or waiting to get enough stones and read them for free. Webnovel is kind. At least, they leave you the choice. Sure, it sucks if you read a lot novel here and don't to want to pay but that's life. Time to choose your priority and novels you want really read. If ever you really want something free, you can try original novels. You'll maybe find your happiness there (let me be shameless and propose you 'Beyond the Mask', my novel xD)
To add, It's rather funny to see people crying that they want to support the authors, but as soon as the money comes back on the table, they leave as quickly as they arrived and forget their resolution to support illegal site.
This point is not addressed to one, in particular, I just wanted to highlight it.
I hope that will have answered your questions.