Why do you need to follow rules?

You need the motive for following rules. Since there are rules for just about everything and you don't specify what kind of rules you're asking about, the answer is very general. An understanding why the rules exist can often help form the motives for following them.

For example, following the rules for scientific experimentation will not only give your result credibility, it can also prevent injury to someone. Following the rules of food production and distribution can definitely prevent someone from getting sick or worse. Following the rules of the road prevent accidents and keep traffic moving safely.

Less dire situations that depend on rules include things like for borrowing books from the library or the rules of a given homeowners association or the rules of conduct at school, at work, or at the mall. These rules have all been set up for safety, and for the numbers of people involved to be able to enjoy the benefits of the given facility on an equal basis.

Rules don't always make sense and don't prevent all injury, but without rules, people would have to struggle and fight for even the simplest activities of society. You see, in addition to all the good behaviors that people are capable of, there's an equal and opposite side to human nature.