Why British English speakers repeat something said to them?

Here in the UK When talking to Britons I see this pattern e.g.

Me: We are going to the zoo next week.
Him: Are we?

Or

Me: We had a lovely dinner last night while Susan joined us at the last minute.
Other people: Did she now? How come she is in London?

does it make you sound like the listener is paying attention or is it a style or is it mandantory in English culture?