Let me define what “affordance” is before I start talking about it. Affordance is really just a contextual potential. Meaning that an object is likely to do something based off the way it looks. The more affordance an object has, the easier it becomes to assume it does (or does not) do something before we even interact with it.
This can be based off of our past experiences or our natural thinking patterns. For example, a door knob affords twisting based on our previous experiences with door knobs. A coat hanger affords hanging, not only by our past experiences, but also by its shape.
Affordance matters because it helps us describe our user interface to our users before they even interact with it. Our users will feel more empowered when they have an idea of what our interface or product does from first glance before the actual interaction. Have you accidentally ever pushed a pull door. You probably felt pretty embarrassed right? In that very same way, our users might feel a similar embarrassment when their expectations of interaction are not met.
In conclusion we can leverage affordance to make our users feel more comfortable, empowered and also make our user interface feel more intuitive resulting in a better user experience.