Stello

So imagine this setup...

Laptop ------> Pa System
Midi Controller ---------> Laptop


Is a midi/usb interface necessary? Is the cord enough?

Or will I be able to control all of my controller's functions and parameters simply with a USB cord to the laptop?

My reasoning is that a sole cord has worked with GarageBand (and I'm guessing other DAWs) without the need of a midi box.

Since the sound is coming from the laptop, not the controller itself, couldn't I in theory bypass the need for a USB interface?

Freebase Dali

It depends on the controller. If the controller has only a midi port on it, and your laptop doesn't, you need a midi-to-usb converter/interface. If the controller uses usb (which means it probably has an internal midi-to-usb converter), then you don't.
It has nothing to do with where the sound is originating. If a controller sends midi data to your laptop via a midi port, how on earth is a usb cable going to help you? It's more than just the type of cable. It's two completely different transmit signaling methods.

Not to fear, though, something as simple as this:

Is extremely cheap. As little as 5 dollars.