Only one other reason, if you were in a 'killing fields' scenario maybe you could 'doctor' one of them to get yourself or someone else out of a country.
If there was a visa in the expired passport that might still be valid? Not sure if that still applies nowadays but it used to be the case (e.g. a US tourist visa given for 10 years which might span 2 passports).
Only one other reason, if you were in a 'killing fields' scenario maybe you could 'doctor' one of them to get yourself or someone else out of a country.