There are two reasons why pearls should be knotted if you are stringing them on silk or a silk substitute.
1 - the knots form a 'hinge' so that the strand moves smoothly and with a good drape. Pearls without a knot will drop on the stringing material and leave an unsightly gap by the clasp - or you will string them tightly to stop the gap and the necklace will be stiff and unbending
2 - in the event of a breakage only one pearl can ever be lost.
Gimp - also called french wire - is the little 5-7mm piece of coiled up wire slipped over the silk to protect it from rubbing wear as it goes around the clasp. It both protects the silk from rubbing damage and gives a 'finished' professional look to a piece of work. Available widely in gold or silver plated and more unusually in other colours
For lots of info on different types of pearls you could look on my website. I just got back from a buying trip to the international gem show in Hong Kong.