In every work place, they are certain behavioral standards (ethics) which should be put in place and exercised as every different working situation requires, by both the employers and employees. A universal example of such ethics will be incentives.
Incentives can be anything, something put in place and serving to induce or motivate. In a work place, incentive will mean inclusion on reward, that motivates effort. If we go by the standard definition of appreciation, you'd find that appreciation falls under the classification of reward or incentives. It may or may not be in a tangible form.
Drawing from the above, appreciation can be said to be the most important act or show of gratitude towards any work done. Asides from the boss's contempt on the work done, which it signals to the worker, it also will help as explained above, motivate workers towards accepting and executing future works to the best of their abilities.
You can say it's of utmost important that every work place practice the act appreciating employees and work done as it will serve as a driving force to compel more work done in the manner expected.