Why does having a lot of rules provide for the community in The Giver?
The safety, the stability, the peace, and all the other benefits that the Community has are all premised on everyone following the rules. Following the rules protects the Community from outsiders, who could bring disease, instability, or perhaps even attacks upon the Community. Following the rules means taking the pills one is expected to take. These pills seem to repress all human emotion, including sexual desire, which has the effect of keeping the Community peaceful and allowing the elders to decide who will marry whom. People who cannot feel emotions are not likely to get into fights or turn against the Community in any way. People who do not have desires are not going to have premarital sex, unwanted pregnancies, or adulterous longings. Nor are they likely to commit crimes, since as long as people follow the rules, the Community is able to meet their basic needs, for food, clothing, and shelter. Following the rules means that everyone has a designated role in the Community, that assigned by the elders, and it is this that ensures that all people's basic needs are met, as each person does his or her designated job unquestioningly. The rules provide for Release, which apparently no one fights against, so anyone who can no longer contribute to the good of the Community is killed.
The entire Community is premised upon every member following every rule, and this is the only way it can provide for the people as it does. If there were no such rules, people would feel emotions that might lead them to unproductive or unpleasant behavior. If there were no rules, people would make choices, and their choices would not always be choices that would be good for the entire Community. They would decide whom to marry, whether or not to have children, what work they would do, what food they would eat, where they would live, and what clothes to wear. People making choices makes life messy and risky. The Community has decided that all the messiness and risk can be eliminated with its rules. Without rules, people would not allow themselves to be killed when they were no longer of use or voluntarily stay within the confines of the Community. They could bring back disease or other threats to the Community, somehow harm the climate control that is in place, or learn that there were places where people actually had choices. What all the rules provide for the Community is the wherewithal to keep it in its safe and sterile existence.