As a member of Science Olympiad, you can get very advanced scientific knowledge in certain subjects. Team members often use high school textbooks and other learning materials in order to become experts on the subjects their events cover.
Another potential benefit of Science Olympiad is that, by preparing for your events, you can greatly improve your writing and research skills. Almost every event requires some research, and the study events especially will give you great practice in researching scientific topics.
High Schools and colleges like to see applicants with extracurricular activities, and Science Olympiad can be a great one to include.