why use SCHEMA? any real-world examples?

I just noticed PostgreSQL's schemas for my first time.
(http://www.postgresql.org/docs/current/static/ddl-schemas.html)

I Googled around, but couldn't find any articles describing WHY or
WHEN to use schemas in database design.

Since the manual says HOW, could anyone here who has used schemas take
a minute to describe to a newbie like me why you did?   What benefits
did they offer you?   Any drawbacks?

Thanks for your time.

- Miles