What is the purpose of both 802.11 authentication and association?

I mean this. Firstly I've coected to my iPhone to my D-Link router : iphone to D-link authentication, association, eapol handshake

And then I've moved to another floor and was reasonably coected to my another ZyXEL router that was closer: iphone to ZyXEL authentication, association, eapol handshake

Authentication frames are always type 0 (open auth) nowadays and Association ones consist of information about NIC of the STA, but while roaming process in all of the types of security (Open, WEP, WPA2) there is exchange with authentication frames and reassociation frames between STA and AP.

So my questions are:

1. Why have not IEEE included information, that is contained in Association frame, into the Authentication frame? There wouldn't have been a necessity in Association frame exchange and thus roaming would have been faster a little.
2. The disassociation frames exist. Okay, I understand: the STA must notify the AP with message "good bye, I'm moving to another girl". But what is the purpose of RE-authentication frames then?