And ironically, one of the reasons why Five Eyes was so valuable is that it allows spy agencies that are banned from domestic surveillance from surveilling their own allies and then sharing the results, or at the very least sharing intel they've received elsewhere.

Snitching on each other's citizens, thereby illegally engaging in domestic surveillance of one's own citizens, absolutely is an integral part of it.

The Germans would have loved intelligence on the Hamburg cell planning 9/11. It wouldn't have just been a benefit to them.