A human being has three things that it needs to keep in harmony: Behaviors, Thoughts, and Emotions. If one of them is out of alignment with the others, there will be a feeling of discomfort called cognitive dissonance until realignment takes place. If it is a one-shot thing, the one that is out of alignment will usually just fall back into line with the other two. But if one of them, Behavior for example, is consistently and persistently out of alignment with a person's thoughts and emotions, then eventually the thoughts and emotions will come into alignment with the behavior.
Example: Door to door field service. When you first become a publisher it sucks and you hate it (emotions). You know nothing is accomplished based on your unproductive Saturday mornings (thoughts). But they tell you what? Just keep doing it and you will come to love it and be more successful at it. And it is often true. When you've been a JW for a year or two and are in the habit of going out every week you kind of like going out on Saturdays "with the friends" and you even start to believe the Thursday night hype sessions when they tell you how great and effective "a shout of praise to Jehovah" you are making.
Why? Because the behavior is persistent and consistent and your thoughts and emotions have to fall into line with it or else you'll be stressed out. It is a psychological survival mechanism.
So why the petty things? Why does the Borganization try to control even tiny things that should be free choice like beards, male jewelry, tattoos, pants on women, holding hands during prayer, etc.? What does that "behavior" influence? Dependence. By controlling little petty things the JW is conditioned to check with the organization first before making any decisions. This drives elders crazy (except the ones on power trips). Some JWs become so dependent they bother the elders every time they make a decision in life. But it benefits the Borganization because it keeps the JW either in a state of unwillingness to try anything outside of the "theocratic routine" that has been set up for them by the society or in a state of perpetual childhood where they are constantly asking for permission from their mother organization and its representatives - with the answer usually, "no."