Why is there set-cookie in every response?

[RookieGz]

Even though there is a cookie in the request, there is a set-cookie in each response. Why not only set-cookie in the first response? Thank you； this my koa-session config:

[RookieGz]

and

[momocow]

FYI,

rolling: false, /** (boolean) Force a session identifier cookie to be set on every response. The expiration is reset to the original maxAge, resetting the expiration countdown. (default is false) */