It's the little things that help us get to know a person and connect with them. If they prefer tea or coffee and how they take it, what kind of food they like, their favourite music, tv show or film,
who their hero is
what sport they like and who they support
what happened that Wednesday in Tesco's
and what they secretly thought of that book everyone's raving about.
The big stuff family, jobs, pets etc is often learned pretty quickly at a basic level. The minutiae of all that paired with the little bits and pieces similar to those listed above are sometimes not shared with them next door but one or the checkout operator.
As writers if we want people to connect with our characters we need a little bit of stuff about them that only their friends and family or sometimes no-one at all would know. I love it when I know stuff about a character the other characters don't or when there's an unexpected twist. In a book/series or tv show I know well I like to predict what the character will do. There is some satisfaction in getting it right but if I get it wrong or I'm not able to work it out unless it is completely alien to the story I will definitely read more of the authors writing.