Because like it or not, first impressions are incredibly valuable. If I am looking at giving you, say, $100k worth of business, and I walk in and it smells vaguely terrible, people are disheveled and the office is a mess, then right off the bat you're down a peg.
Assuming you have a competitor that is clean & neat, with employees who are presentable, all things being equal I am more likely to choose them. You & your employees might actually do better work, but you're fighting an uphill battle from the get-go. And that to me is one thing that a successful business doesn't do: immediately put themselves at a disadvantage.
And it's not just customers or investors. It can be vendors, partners and, quite frankly, other employees. Ultimately it's a place of business and there are certain standards that need to be set that otherwise some employees might not set their own appropriate ones (whether it's smell, hygiene, shabby dressing, what-have-you).
To clarify, I'm not saying you have to have everyone in suits to have a successful business. I'm just saying that someone should at least be able to enforce a basic level of present-ability throughout the workforce. Even if that just means "Hey, ABC, you gotta start wearing deodorant to the office."
