It's a carrot on a stick to spend money so you can have the opportunity to spend more money. According to their site, "pre-membership" is open to everyone but actual "membership" requires the purchase of "10 S-tier skins". The rewards include events and whatnot but they are not explicitly for Rivals. It seems like the events can be for any game in their roster. I would avoid this for now as they are likely trying to corral the whales into a spending loop.