Understanding ethics can help in making the right choice in a complex situation. For instance, if you go with act utilitarianism, you choose the action which maximizes a positive effect. If you believe in rule utilitarianism, you use the principle of utility (maximizing a positive effect) to set rules to generally abide by such as keeping one's promise. But, there could be situations where following the rule gives rise to a negative effect. This is similar but different to determining rightness by examining the act itself. Then there's virtue ethics such as those of Socrates and Aristotle. There's also the role ethics of Confucius.

So, it's often good to examine a complex situation from the viewpoint of multiple ethical systems and make a decision based on that. That's what courses in ethics do. They teach various systems and present situations to examine.

A lot of people might go with consequentialism, which is basically, "The end justifies the means." Utilitarianism is essentially a subset of this.

So, yes you have some ethical system that you follow, but if you're only looking at a situation from that viewpoint, you might miss out on an alternative course of action.