[Swizec]

OpenAPI is only useful if you intend for your project to become bigger than what fits in your head or if you intend to keep a project alive for long enough to start forgetting things you did a while back.

[jlengrand]

Or if you deliver an API and any of your users / customers want to use anything else than a raw API. IMHO the top value of OpenAPI isn't for you, it's for the people who will use what you make

[salomonk_mur]

Agreed. Most of the value comes when people other than you need (or will need) to use it.