First, I am amazed at the rude attitude of some. This is supposed to be a place where someone can turn for helpful answers to a problem, not a place where they run the risk of being mocked for their problem.
That is completely counter to the spirit of these boards.
However, since the question was asked, I'll answer.
I have two teenagers and I often need to speak to them during the day or get messages either to them or from them. Their schedules can change from day to day or hour to hour as game times are updated or activities rearranged. That in turn can affect who or how they are being picked up. Sometimes even where they are being picked up.
Typical teenagers today have flexible schedules so pickup plans must be flexible.
Further, my wife and I have schedules that fluctuate sometimes. Sometimes we are late. At our kid's high school, you pick up in different places depending on the time or the sport. If you are coordinating between families it gets even more complicated.
And before you suggest just calling the school and telling them to leave a message, let me assure you that in our school of 700, that would be laughable. Teachers and administrators don't have time to track down every kid whose parents are running late.
These examples just relate to pick up. There are countless other situations where parents need to communicate.
It isn't about being controlling or stripping a child of freedom. Its the opposite. With reliable access, my child, has the freedom to change plans and be free to choose what he wants to do. If we couldn't communicate, my kid would always have to be ready to go at the time I was expected. The reverse too. If my plans get screwed up, why get the message to my kid so they can go hang out with their friends or get a different ride, rather than sit bored waiting on the front steps?