In an ideal world, you will always get exactly where you are supposed to be at the time that you are expected to be there. In real life, delays happen. Perhaps you got tied up at the office, in court, in a deposition that went longer than you expected, etc. So sometimes you need to let someone know that you are running late but you are on the way there. You can always send someone an email or a text that says exactly that, but the iPhone gives you some other options that might be even more useful for you.