It is a little, unsettling, when they set up checkpoints to make sure people exiting the country are authorized to do so. I'm sure people can state good reasons, but it feels very East Germany-est.
However, it's quite common in Europe and other countries. Every time I fly out of a European county, my passport has been check to confirm that I actually left. Same with several Asian countries.
Apparently the UK doesn't do it, but other EU countries do.[1]
[1]https://www.flyertalk.com/forum/checkpoints-borders-policy-d...
The UK has the ID document details checked by airline staff (or their contractors, like handling agents) with document details passed on to the Border Agency.
It sounds like they don’t really have any automation to relate departures with overstayed visas.
These tracking systems end up looking at everyone, because anyone might be an immigrant.
Again, is this a real, rampant, dangerous problem? Probably not. But it's a real use-case (so they don't have to give fake reasons) for state-wide surveillance. Since it also claims to target foreigners (which by definition aren't voters), most locals/voters/politicians won't care, thinking it doesn't apply to them.
Not that I'm defending this, I think this level of surveillance is extremely dangerous. And the most obvious thing is that they'll slowly and quietly expand this to everyone else.
It's also a "permission to enter" some other country. But without it, you won't be flying anywhere.
Then you've got visas, which are "permissions to exist" in a certain geographical area (within a certain time-frame).
We've all had a default assumption that we can get a new passport any time, but it's actually never been guaranteed.
Our current situation might be marginally better than "double borders", but you're probably aware we're headed in a police statey direction all over the world.