The process of sanding wood creates scratches on the surface while flattening and removing unwanted wood. The higher the grit rating, the smaller the scratches.
I start with 80 and progressively use 120, 220, 320, 400 and 600 on my products to get a smooth, polished surface, ready for oiling and polishing. It only takes about 3 minutes to switch a belt, so you can get away with a single machine. I currently have a row of five sanders, each with it own grit, to speed up my production during busy times. 600 is [almost] always done by hand.
Skipping a grit may save you a little time in switching belts, but will leave the deeper grooves of its predecessor with flattened tops. They will not be apparent to the eye until you apply your finish, leaving a dull (often darker) or unexpected appearance. Counting on your finish to smooth things out rarely works.
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