The U.S. government also mandates that official government e-mail accounts be used for official business, since we have laws that require the archiving of official communications (to avoid corruption, etc.).
For example, there was a controversy a few years ago when the head of the EPA, Lisa Jackson, was found to have been doing EPA business using the e-mail alias of "Richard Windsor", in an apparent violation of federal records laws.[1]
There are also industries like finance where laws mandate that e-mails must be archived for a certain period of time to make them accessible to regulators in case improprieties are alleged. Many other businesses also want to keep archives of employee e-mails in cases patent disputes or other lawsuits arise.
[1] https://en.wikipedia.org/wiki/Lisa_P._Jackson#EPA_Administra...