Two key attributes that professionals attain are their skill set and their experience with that skill set.
Experience is valuable in many aspects:
As a professional gains experience in using a skill, that professional improves their ability to use that specific skill, measure either by reducing the time tasks take, or increasing the quality of that task's output (or both). See Wikipedia's atricle on the Experience Curve Effect for more information.
Increased experience often results in better judgement about when to use skills, or which skills might be most appropriate in given situations.