I think the main thing about calling objects and cars is because if you're in a true paceline then you won't be able to see, well, anything other than the hips and bum of the one in front of you. So if there is something to see, they need to alert you, likewise, the last person on the line will be the first to know about a car and can oftentimes be the only way the people up front can be prepared for one coming up.

Calling out every car, though, on a busy road... well it does get old. Unless it's something large and shoulder-hogging like a trash truck or schoolbus...

Anyway glad your ride went well!

K.