The commonplace hoodie with screen printing on it, for example, is at once a statement of a casual attitude towards appearance, and at the same time, with its decorative element, a call to be noticed.
Certainly there are always extenuating circumstances that may make, "the perfect coat" a temporary impossibility. You're going on a special date, and it starts raining hard. Suddenly your old tan trench coat gets hauled into service, so your other clothes stay dry. Or more likely, and this would be for most people, you just plain don't own something that is the ideal first visible word in your Attire sentence. So, you make do with the closest thing. Or perhaps you're one of the army of folks who don't think about such things a jot. That, of course doesn't alter the fact that you're still making Attire remarks, or that others will see, and interpret them in their own way.
And of course, we get to the title of this post, and a meaning implicit within those the words. More than just protection from the elements, an outer garment is like the furthest ramparts of a castle and keep. Its the surface layer of protection from others, or our first announcement of who we are, or wish to be. Some use a coat as a blazing firey signal of Here I Am!
Whichever reason, or whichever of the multitudes of other psychological motives we might bring up, it remains that this garment, this flexible shell, is one of the first symbols that states, come here, or stay away, to the people we pass in the street.