A passport copy can often contain sensitive information such as date of birth which can be used to access other sensitive information such as bank accounts.
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Mobile Passport Control (MPC) allows eligible travelers to submit their travel document, photo, and customs declaration information through a free, secure app on their smartphone or other mobile device.
Many countries have laws requiring foreign visitors to register with the local police. By taking your passports the hotels are doing this for you (and it's not their decision - it's the law - they have no choice).
As per Spanish law, accommodation providers and hosts on platforms like Airbnb and Booking.com are obligated to collect specific information from their guests. This includes your full name, birth date, nationality, passport details, and the country issuing the passport.