[StopDisinfo910]

Keeping track of which of your citizens are outside of the country. Ensuring the state knows you are a citizen and should be treated as such.

France had a weird issue recently about the media talking for ages about someone who committed a crime while the state had asked for him to be deported months before on the basis of his foreign passport and it took weeks for someone to finally notice that the guy was actually French. It made the police looks clownish.

[dcminter]

That's your guess. The UK authorities have never given this reason.

[StopDisinfo910]

You were asking for legitimate purposes. That's some of them.