Personally, I just want to know for what purpose they're used for.
I would ask: why would they choose to have your cookies automatically expire at any point?
Stack can clear the cookies when it's appropriate for them, such as when you log out.
Your browser can clear the cookies when it's appropriate for you, based on your privacy settings, storage requirements, or whatever else you consider important.
Newer APIs, like Storage and IndexedDB, do not require (or even support) auto-expiration dates, beyond the high-level choice of "indefinitely" versus "session". Only cookies require this. Choosing an arbitrary date in the far future is a good way to ignore the requirement.