It makes her happy, delirously, ecstatically happy, to let go, to let another have all the power and make all the decisions. There's a curious freedom that comes from knowing you have no alternatives, no choices, and can only react as your own true self to the situation you find yourself in. It's part of the attraction of such things as high-speed motorcycle riding, skydiving, and the other things which appear so foolish when viewed from outside but give us such joy and triumph when we do them. The trappings and rituals of slavery are very important, perhaps more important in this society, where the slavery itself must be an intimate secret, to the feeling of ownership and domination to the slave. In John Norman's Gor books, one of the most important trappings of slavery is the collar.
Collars serve three purposes.
First, to identify the collared as a slave. Second, to provide a convenient method of affixing bonds to a slave or of securing them. Third, and most important, they impress upon the slave her slavery. The weight of an honest steel collar is lighter far than the million social chains and bonds of an unhealthy society, but its realness, its weight, and the sense that it will not come off save at the whim of the owner, are terrifying and precious to the slave girl. My slave girl has a collar, handmade by me.