I'd like to share the customer perspective as an end user. Appearance matters.
Polos are better than t-shirts. Button downs are better than polos.
Denim is better than Walmart cargo pants. Khakis are better than denims.
When a tech shows up looking professional, I give them that until they prove otherwise. When a tech shows up wearing a faded "This Bud's for You" t-shirt and cargo shorts, it gives the impression that they have no contact with the employer apart from having their jobs emailed to them, or that the employer doesn't care.
When a tech looks neat, I assume his gear is organized and I imagine the shop and the operation are the same.
It's also important to stress professional behavior. When a tech is unprofessional in his/her behavior (profanity, lax work ethic, loud voices, sloppy work summaries), I assume the entire organization is, as well. After all, what else have I got to go on?
Of course, I realize that it's all window dressing, but it also tells me who doesn't care enough to put a little effort into window dressing.