For beginning users of worldwide network sometimes there are difficult even simplest operations. Besides, the software changes quite often, and users not always keep up with updates. In such conditions of complexity with reading of the entering e-mail are very clear.
You want to tell the friend the e-mail address that he could write you, but find out that you cannot make it because you do not know own address. Quite seldom, but it happens. There are several ways of definition of own address e-mail.