Are there any good reasons for the fake ‘X is typing…’ delay of ChatBots?

Is this is something that will go away when the 'coolness' wears off or is there really a grounded reason; like people experience it as freaky when you get an instant answer; that it gives the user a similar feeling like the uncanny valley?

I really get impatient from it, but maybe it works for 90% of the people - so I'm looking for more theory around it. Is there any theory about this?