What is /System/Library/CoreServices/WiFiAgent.app?
WiFiAgent.app is part of OS X and is the agent responsible for all things Wi-Fi in the GUI.

This includes everything from managing the menu bar item to showing dialogs for entering the wireless key of the network. The agent runs in the background on all Macs with a wireless service.

Here's a converted version of the WiFiAgent.app Localizable.strings which shows some of the features of the agent through the messages that can be localised.

The reason why WiFiAgent.app is shown in Finder as "Wi-Fi.app" is because of the InfoPlist.strings file defining CFBundleDisplayName:

{   CFBundleDisplayName = "Wi-Fi";
    CFBundleName = "WiFiAgent";
}


Contents/Resources/InfoPlist.strings