I expect the usual reasons will apply. One of the main functions of the “libro” is proof of civil status and family relationships with children, along with the relevant dates. It is / was needed for many official processes, such as starting your kids at school.
However, having said that, let’s not forget that the “libro” is being phased out this year. All these personal details are being placed into electronic files, and so the authorities will have immediate access when necessary, without asking you to produce the “libro”. So, while making life a lot easier bureaucratically speaking, it could also make some peoples lives a nightmare if the details recorded are incorrect. I await with bated breath to see if the authorities manage to mess my life up (again) or not.