alt Hacker NewsIt looks aesthetically nicer. It was also a bit of a signal that someone took pride in their work and so helped that way. It's a bit like whether your tradesperson cleans up after themselves. Technically sweeping up the dust after installing a kitchen cabinet doesn't actually mean anything for the quality of the kitchen cabinet installation, but in practice putting the effort into the presentation correlates with putting the effort into the actual work.
I understand the sentiment with the tradesman example. I would argue that there are a multitude of ways to care about or take pride in one's trade and emdashing the floor is just one of them.