Quick travel question: Checking in early and skipping the ticket counter

[TipsyTraveler]

We fly from Detroit to Heathrow a couple of times a year via Delta or Virgin and always have to check in. It may be because DH is a British citizen with a green card and I'm an American citizen but we have to have our documents checked at the counter both ways. I've always thought they had to make sure your passport wasn't close to expiration.

I don't know? We do online check-in the day before and then our next stop is security with passports in hand and our boarding passes pulled up on our phones. I just doubled checked with my husband to make sure I'm not misremembering. The only time we got bumped out of the security line to have our documents verified at the counter was on the last leg of our most recent flight coming home from Heathrow. But, I've also had the TSA send us through with unsigned passports after pointing out that our passports were invalid because of that. So who knows whose not doing their jobs properly and allowing us to sneak onto planes.