Knots that are tied between pearls to keep them from rubbing against each other and damaging their nacre, the iridescent layer we see on the surface of pearls.
Knots also keep pearls more secure by acting as security stoppers--if a beaded strand breaks you might lose one or two but the knots will keep most of the pearls from falling off the strand.
Inexpensive fashion pearls are not typically strung with knots between them.
Celia Martin's pearl knotting tutorial is a step-by-step photo gallery that explains how to make pearl knots. It will give you a better understanding of the time and effort that's involved in stringing pearls this way.
How to Clean and Care for Pearls