It's so kids' stuff to harp on punctuality, but here I go anyhow.
Lateness costs money
Imagine a meeting with ten people. The meeting cannot begin until everyone is there. These people cost the company about a hundred dollars an hour to employ. The meeting starts fifteen minutes late. The price tag on that one time delay is $250. If that's a weekly event, it's about $12,500/year. Once daily? $62,500/year. I had that meeting at least three times a day every day while at Microsoft. Microsoft spent $200,000 for my team to wait over the course of a year. Just my team alone.
Lateness costs productivity
The same scenario as before yields the following lost work hours: 2.5 hours vanish for a one-time event. A little over three work weeks vanish over a year for a once weekly delay. That delay daily vanishes about a month (distributed across the team) during a year. My team lost about a full quarter of productivity in a year. By waiting. I should mention that my team was directly responsible for helping (or hindering) all other groups release their software out to the market.
Lateness costs good will
It may just be adding insult to injury, but this insult can lose sales, impair relationships, and sometimes even scuttle whole businesses. The signal that you send by making me wait - knowing that it costs my company and knowing that it hampers my productivity - is that your time is above those concerns and above my own concerns as well. At best, being late can be forgivably rude. Being on time is easy, so it had better be worth it to wait.