"What motivates a willing, or even reluctant wearer of a chastity device (and accessories if applicable), to give up control of what some might consider the most important part of their being?"
Hello. I've asked myself question this same question for a long time.
Traditionally, chastity belts are seen as highly protective. They protect the wearer from assault, rape, unwanted pregnancy and STDs. I think safety is of a very high importance to many belt providers and the safety aspect of chastity belts is undeniably true.
I think self-mastery and personal development are also motivators. Sexual urges are strong and can override good personal judgement. A chastity belt more often than not ends up protecting a wearer from themselves. In this way pre-commitments can be made to avoid ill-judged sex, bad or shallow relationships, and the rabbit holes of masturbation and porn addiction. A chastity belt can serve to channel the sex drive into productive avenues such as study, work and social advancement. It may seem old fashioned, but chastity is a virtue: it's a good thing to seek in of itself!
As you happily report; chastity belts strengthen and deepen loving relationships. They are one of the strongest possible signs of trust and commitment. Preserving virginity, saving yourself for a life partner, and long-term fidelity have huge value. I will never have a sexual partner other than my wife, and this means something special.
I am always saddened when I read of negative feelings towards belts. If offered at the right time and in the right way a chastity belt should elicit feelings of pride, maturity, and security. Wearers should feel protected and respected.
As chastity belts can be a little uncomfortable, difficult to clean and can sometimes interfere with activity, I see no reason not to compensate wearers with financial support, increased freedom, praise or even gifts. However, the choice to wear a belt must be made freely. A wearer must consent to a belt's installation and a large part of this consent is the knowledge that another must then give consent for its removal. (-That's the point, isn't it?) I've read on here that wearers report feeling intrigued by the whole idea and it's certainly true: wearing a chastity belt is an unforgettable life-experience!
So; to answer your question, I think chastity belts protect the wearer, their health and their relationships. As always, if the motivation is love then things will work out well in the end.