For my husband's British Passport application, they explained he'd need to give the names and dates of birth of his paternal and maternal grandparents. The difficulty was that they all have the date of birth of January 1st and an 'approximate year of birth' as no-one knows when they were born! The chap during the interview was very kind, and understood the problems. But he said all these details would be stored at the Passport Office and would be used as means of further ID, in case anyone stole the passport.
It's interesting that in my husband's country, there can't be much personal information stored, as no-one knows much about their own details, and no-one actually even has an address. It's usual to ask the 'chief resident' of a neighbourhood to show you the house of the person you're seeking. There isn't any postal service. And no utility bills. It's so different here in UK.One can be traced by any number of details.