Blimey! I think that @FreakyFridays missed the early helpful posts which distinguished between manners and etiquette.
Chewing with your mouth closed - good manners (nobody wants to see your mushed up food)
Holding the door open for somebody - good manners (thoughtful because you're making sure that the door doesn't slam in their face)
Taking shoes off to go in someone's house - etiquette (cultural norm)
Keeping shoes on to go in someone's house - etiquette (cultural norm)
Asking the host if they'd like shoes on or off - manners (showing consideration to host)
Not commenting on whether guest has automatically kept shoes on or has taken them off even though they've chosen the option that you don't really like - manners (avoiding embarrassing the guest).
This wierd (to me) custom of breaking off bits of a roll and then just buttering the piece that you are about to eat is most certainly etiquette.
I've got pretty good table manners and I'm aware of and use certain conventions (hold my knife and fork "correctly", can lay a table, know that you work from the outside in with cutlery, put my knife and fork together to show that I've finished eating etc) but have never heard of this before this thread let alone see anyone do it.
I've never been to a formal dinner so concur with those posters who guess it's something which posh people do. To me that's Upper Middle Class and Upper Class.
It just goes to show that if you are brought up to follow certain conventions and if people around you also follow those conventions, you fail to realise that they are just the cultural norms of the society in which you are mixing and you end up conflating etiquette with good manners.