> But why can't those two just tell the team they're blocked via email/IM?

I agree with you. You're better having this kind of stuff on a planning board so you can easily see the status of every task with all the information in one place. Why do I need to know the reason for a task being blocked when I'm not working on it and can't do anything to help?

They can, and if that works for your team then go for it. Generally, forcing people to talk to each other as human beings tends to get things resolved much more quickly than easily dismissible messages/emails.