Your pitch is important, and tone of your copy is important in framing and expressing this. For example, if you're asking me to download an executable you need to gain a certain degree of trust on behalf of the user. Trust is an emotion. Good art & copy can go a long way to building trust.
Branding is really the emotional connection you make with your user base, and emotional connections aren't rational things. This is why excellent products sometimes lose to less functionally valuable products - because we've distilled the rational meaning down to something which is irrational.
To "like" Apples products, for example, is not rational - it's emotional. The meaning behind Apples brand is more than the functional value of it's products. It represents something different to each of us, even though we're all using the same product. For some people, Apples brand represents a part of their own identity - this is about as emotional as it gets. Apples brand communication has touched these people in a strong way. Do you think Apple could have built such a loyal following with a MS Paint logo and terrible copy?
To put it another way, branding could also be thought of as packaging - and it's well known that packaging is extremely important to product success.