Commuting is going to or returning from your place of work. The issues of securing cycling gear, bikes, lockers, showers, rush hour, and most probably reduced light much of the year are unique (though not exclusive) to commuting by bicycle. A similar theme would be cycling when primarily used for utility to get the cyclist from point a to point b as opposed to exercise, recreation or sport. Yes all reasons can be performed on the same bike though at different times. It may be just me, I don't consider bike messengers or pro-cyclists to be commuters as their place of work is in the saddle. They are simply professional cyclists.