There are few good excuses for being late, and sometimes the best thing to do is to cancel if you can call with enough advance notice. But being late shows your guest, client, friend, potential customer, that you don't respect their time, or that you are very disorganized. Be early if you need to be.
I recently went to support a business acquaintance by attending an event she put on. She was an hour late. That meant, by the time she was ready to begin her product presentation (after an initial introductory portion of her event), I had to leave. Someone else did to. So she lost two potential customers because we had prior commitments planned for after her function.
Remember to consider that other people are busy too, and let them know when your presentation begins and BE THERE. Also, let them know when it will end, so they too can tend to other obligations.