It is a requirement by the law. All players in Spain need to have release clauses inserted into their contracts due to some kind of work legislative.
A lot of players have extremely high clauses - the clubs know they cannot be triggered and are protecting themselves like that.
Barcelona is extremely careful after Neymar's case, whose clause was put to 200 million - also so that no one can trigger it (it was unimaginable when he signed the contract) - but PSG did a few years later.
Thanks, that explains it. I thought it seemed bizarre thinking that someone would actually trigger it, and it must just be a PR stunt. I hadn't realised it was a legal requirement. This makes perfect sense now. Although it does seem almost like they're exploiting a legal loophole a bit... if a fee is going to be set at such a level that it will never be triggered, then it can't practically be considered a fee at all. Although as you say, who knows, no-one ever thought Neymar's would get triggered.