Interaction design laws
Progressive disclosure
Show people only what they need now, and reveal advanced options when they ask for them, so the common path stays simple.
Most people use a small part of a product most of the time. Putting everything on one screen makes that part harder to find; hiding the rest one step away keeps it out of the way without removing it.
Decide what's essential with research, not guesswork, and make sure the way to the advanced options is easy to spot.
In practice
A print dialog that shows copies, pages and printer, with 'More settings' for paper type and margins.
Common mistake
Hiding something most people need behind 'More', or nesting options so deep that nobody finds them.
Where it comes from
Used in the earliest graphical interfaces of the 1980s to keep them approachable, and made a standard pattern on the web through Jakob Nielsen and Nielsen Norman Group's writing.