In BC, Canada bars and servers can be held liable up to millions of dollars for DUIs and drink driving accidents when they over-server someone.
The mandatory government licensing course for all who server alcohol, Serving It Right, makes it clear and even cites an example where a server sent someone home in cab. The person them left their home in their car, and the server was fined.
You usually block people for good audience sight-lines and sensible character interactions. It can also communicate relationships non-verbally.
And you want to avoid awkward shuffling as they enter and exit the stage.
Blocking is important in film as well, but less so due to close ups, shot reverse-shoot, and editing.