Q: What does "Use CallBar for all outgoing calls" do?
A: When this option is ON, any call made from any application on the device is handled by CallBar: CallBar banner appears dialing the call, as if you had dialed it from CallBar's dialer interface.
This way, you have your device available for anything you might be working on while dialing.
If this option is OFF, an outgoing call will bring up the Phone application's interface. This does not allow you to browse around the device unless you exit the Phone application, leaving you without any in-call options.