Nowadays, in many companies, it is normal to be very reactive and respond to emails very quickly. However, when we read Cal Newport's book, we realize that instantaneity is not so good because it encourages us to spend much more time responding to electronic communications.
When an employee interrupts what he or she was doing for 5 minutes to read his or her e-mails, it will take some time to get back to the level of concentration he or she had before interrupting his or her work.
This is also true in leisure. If you interrupt reading a book to reply to a Facebook message or answer the phone, you are less focused when you start reading again, it takes several minutes (or more) to be fully focused again.