Answer:
1) If you happen to be struggling in their class you could come to them without feeling embarrassed. You could ask them about helping you with an assignment, understanding the material, and even mentioning if they offer tutoring as to help you better understand the information you're learning.
2) If you happen to be having issues at home you can ask for help because you know that you can trust them. It's hard to find an adult to rely on other than your parents, but if you have a relationship with one than you can learn from them and ask for advice when you don't feel comfortable confiding in your parents.
3) If you're in High School you can go to them about college questions, such as asking for a recommendation letter if you plan on going to college. Also, teachers go to college, so if you have parents who don't have that experience than you can ask them instead. For example, what colleges look for in your essay, colleges that will support your career path, or even scholarships that you should apply for.