subscriptions keep the developer honest with making a game that continues to entertain the users who have paid for it. It ties the developer to the tenet that their game must be good not just for the first few hours you play it, but it has to be good at the 1000th hour too. It keeps revenue steady so they're not inclined to create these infrequent - large - bug ridden updates to excite new users. It allows them to fund constant content. It basically would have killed the insufferable grindish half-baked game mechanics years ago because players would be able to vote with their subscriptions and fdev would have had to respond.
Which is one reason why FD didn't do it that way.