Ethical theory is an interesting subject in its own right. Even if studying it doesn't make you a better person, that doesn't make it useless, any more than studying abstract areas of math or physics is useless.

That being said, some work in practical ethics has had a huge impact. Peter Singer's work on animal ethics started the modern animal rights movement, and his essay "Famine, Affluence, and Morality" has inspired many people to donate a lot more to charity than they otherwise would.