It's a thing because a lot of creators and agencies use automated software that pulls the links from google searches and web crawlers and sends out automated takedowns. Links would be dead in a matter of hours in some cases. Putting the links behind a wall like this ensures that
1) the person getting the link has to be a logged in member of the forum
2) has to click a little button, which a web crawler won't do
3) doesn't require any sort of investment or special privileges to you
Basically it keeps the bots out and keeps links alive longer all while bringing content to you at no charge