Designing your system for extensibility creates an open-ended system that can have surprising upside.
Designing a system for extensibility that does not quickly become a quagmire or a security problem is hard!
It requires taste and judgment to design it properly. A rare skill!
A kind of meta-engineering ability.
Sometimes a system that wasn't designed to be extensible can be extended.
E.g. some very popular games have very active game modding communities.