Session_cookie_lifetime - why the default value is the default 604800 (7 days)


We are just wondering why the session_cookie_lifetime cookie life time set up in System Settings has this specific value 604800 (7 days).

Our goal is for the browser to set the PHPSESSID cookie with expires = “session” rather than in 7 days time.

default (session_cookie_lifetime = 604800)
preferred (session_cookie_lifetime = 0)

Setting session_cookie_lifetime to “0” does that but the question is why is the default 604800? Are there any issues to consider by setting it to 0?

many thanks guys!