During the 2000s, following links for a forum or blog was way too expensive, so they had black lists of dirty words to avoid porn sites spaming and get juice during the page rank golden years where any back reference mattered.

Hence it was just easier, to avoid the filters, to create non blacklisted domain names with redirections.

Then another trick was to write a perfectly legitimate page, get google to index it, then redirect that page to the less legitimate page. Because at the time Google refreshed once a week (or a month...), you'd get plenty of traffic and revenue for long enough to be worth it. If you sold niche porn and viagra, that is.

Another one was just to setup fake sites with different URL schemes with stats on them, and get a regular update on which URL formats were getting the best hits. At the time URLs where very important in getting points. Then you would regularly update your most important sites URL scheme accordingly, several times a year if needed.