We writers need to write, just as runners need to run and musicians need to play. It is a psychologic, if not emotional, impulse.
Those whose job requires daily essays likely find that their work fulfills that need, though many end up writing novels, nonfiction tomes or autobiographies as well. Others, especially those who have signed agreements with publishers, are required to produce. Then there are the rest of us.
Blogging fulfills my need to write but, as many bloggers find, subject matter does not always arrive on schedule. Since we are generally unpaid scribes, we are not under the gun to publish our reports or opinions. Nevertheless, the urge persists and we end up writing more than we release. Indeed, bloggers may have a number of drafts waiting to be finished and published; many never see the light of day and are discarded. Non-writers may feel this is a waste of time and effort but writers know better; we must satisfy a basic need.
See also: The Therapeutic Nature of Writing