It's a distinction writers need to bear in mind. Dialogue, as written in fiction, has a primary function of entertaining the reader (or viewer or listener). Let's suppose you have a scene of a mother and her two adult daughters talking about family. It's serving a purpose, that of showing how the family bonds are being cemented. But once that's been established - and it should happen pretty quickly, within half a page I'd have thought - the scene will need a new purpose.
The new purpose could be anything - a hidden agenda for one of the participants, preferably a hidden agenda for ALL of the participants, which they're trying to promote. Secrets, long established grievances, revelations for the future - all suitable purposes to move the story on and make it interesting.
In real life, we're quite happy to ramble on all weekend about family stuff, but no reader/viewer/listener is going to put up with that. Dialogue in fiction constantly needs new purposes or it won't work.