Piano regulation is a continuous refinement of the many points of adjustment within the piano action. If you took a grand piano action completely apart, you would end up with thousands of individual components. Regulation brings all of these parts to work together efficiently to create a responsive piano. A full regulation may take a day, but individual adjustments may be made to bring drastic improvements to your piano.
As pianos age, the wear on the parts will throw the regulation out of adjustment. The felts, leather, and punchings compress as the piano settles in, so adjustment will be necessary. New pianos also require regulation after a year or two, as the parts begin to break in. Most factories try to compensate for this by submitting the piano to hours of playing, but this cannot fully compensate for how the piano will eventually settle.
A piano can function with the regulation terrible out of specification, but the playing may not be enjoyable. If you feel like you are working too hard to make the piano play the way you want it to, regulation may be a solution.