Why does client authenticate before associating with the AP. What does open system authentication even achieve. I fail get an accurate reason from my web searches.
In a wired network, you have a one-to-one mapping between the client device, the network cable, and the network (via a switch port).
In a wireless environment, it is possible (and indeed common) that multiple wireless networks exist in close proximity to each another, so you need a way of specifying which network your client device will connect to. Open system authentication is the simplest method of doing this eg: specify the SSID of the network and your client will connect only to that network.
Unlike wires, where you can control who connects to what, wireless connections easily penetrate walls and property boundaries. In your network, you'd want to control who can connect.
Additionally, wirelessly transmitted signals can be intercepted everywhere in the vicinity - you'd want to encrypt the traffic to protect sensitive data.