Speech uses the same part of the brain as thought apparently, so making someone say words prevents them from thinking during that time.
Only slightly related, but this is why I listen to audiobooks every night to fall to sleep. If I'm listening to someone else, I almost literally can't "think" so my brain chatter is basically eradicated and I fall asleep. I know it's not the same for everyone, but I basically turn into a mindless automaton if I'm actively listening to someone else as I'm an extremely verbal thinker.
Not that I wouldn't know what you said, but I won't have a deep understanding, I won't be able to use what you said to get work done. This is especially bad with phone conversations.
But if you write it down, then we're talking. I can digest written stuff extremely well.