Comment by TruePath

To clarify the point in the other reply -- imagine it sent one packet per keystroke. Now anyone sitting on the network gets a rough measurement of the delay between your keystrokes. If you are entering a password for something (perhaps not the initial auth) it can guess how many characters it is and turns out there are some systemic patterns in how that relates to the keys pressed -- eg letters typed with the same finger have longer delays between them. Given the redundancy in most text and especially structured input that's a serious security threat.