They're not compulsory but every club seems to want to use them so as to protect themselves from a possible tapping situation especially with young players. But as is well known, they mostly end up working against the owning club unless they've done their work properly by renewing the contract annually as the player progresses. At big clubs they protect themselves even better by making big clauses and there's another issue in that it's not just the buy-out clause the purchasing club has to pay out. The moment you activate a buy-out clause, the buyer is responsible for the tax on that purchase and they have to pay the Spanish fiscal office themselves which at those prices stands at almost 50% and that's a lot of money especially on higher prices.