You say "Hi!", I'm in the middle of wrapping up something. 8 minutes later, I have time, I say "Hello, what's up?". Unfortunately, during these 8 minutes you moved on to doing something after waiting for 30 seconds. 12 minutes later you see my reply, and say "Can I ask you something about this Foo module"". Again, during these 12 minutes I moved on to something else, blablablabla.

Had your first message been "Hi! In the Foo module, do we really need the function Bar to be public?" you would have been all set after 8 minutes.

A chat, like email, is just a tool to get shit done.