Now that voice calling is almost free even internationally and you can email your parents at anytime, I do not know why this would be a useful service? Calling someone or writing a letter or even emailing directly has a more personal touch than using a service like this which emails your relatives to write about their life.
You're absolutely right, you could do this yourself. In practice though, I've found that people really like the questions we suggest, and the fact that we automatically send the questions, reminders, etc. It just makes things easier, and therefore more likely to happen.
As for the personal touch, the question is really just a prompt to trigger a meaningful conversation which might otherwise not have happened.