Thinking of a chastity belt as something you put on people you do not trust completely misses the point of a chastity belt. There are a few situations where a chastity belt can be used to recover damaged trust, for example as a form of reconciliation proposed by a partner who has confessed to cheating. Otherwise, if the keyholder has no trust in the wearer it is usually because they forced the belt upon the wearer, and you know my feelings on forced device wear.
No, chastity devices in a relationship usually serve to enhance a commitment. We are all humans and we all make mistakes - most of us would not make a big mistake like having sex with another person behind our partners' backs, but in some relationships, that commitment extends further, e.g. no masturbation without your partner's presence and support. And when the urge for that strikes, you might be strong enough 9 times out of 10, but that tenth time, your willpower may falter, and you will be happy to have a reminder between your legs that tells you not to do that, and makes it harder to do so if your brain is not listening to that reminder.
Other people see it mostly as a reminder of their partner's presence. If the partner has a key for your chastity belt, whenever you feel the belt between your legs, you feel the intimacy of the person who put it there.
