It is a time-consuming and often tedious process--adjusting slight movements and footwork--that can occupy a student for the rest of their life. It requires dedication and love; it requires the drive to improve above and beyond the average individual who steps on the mats.
Bad habits form and need to be corrected; new insights are delved into; a new teacher has a new take on an old technique. It all comes down to small things, and small adjustments of one's perception of the art. It is adding the detail to a painting done in big brush strokes. (It was Picasso who said a painting is never finished).
Refining, to me, is an enjoyable aspect of learning. It means you already have a decent grasp of the concept being studied, and the slight nuances can be the difference between a technique working or not working in real life. It also reinforces what you already know via repetition--a major part of budo in general--and allows one to learn further by trial and error.
However, it is the time and dedication that separates the strong martial artists from the adequate or weak. And time is not something just anyone will commit to something they think they already know.
But each day we must have beginners mind.