Internal experience is an input to your mind (that’s how you know about it), and what your mind perceives affects what it does. The better question is: how can what you experience possibly not affect your actions?
The need arises when you consider the complex social systems humans leverage for survival — our ability to navigate interpersonal conflicts and engage in cooperation relies on theory of mind and our tendency to perceive ourselves as individual free agents empathizing with separate free agents.
By definition, internal experience is something you perceive, and what you perceive informs how you act. Therefore internal experience affects outward physical action.