Singledad, being in an extracurricular activity is associated with getting higher grades. The correlation is strong, and one of the main arguments in favor of providing a variety of extracurriculars for students.
Being on a yearbook committee (you mean designing the yearbook, I assume) shows that a student has practice working with graphics programs and page layout, can be depended on to go out and get the pictures and/or interview for captions they need on their own time, and can work together to produce an appealing publication. That says a little more than "I showed up to English class for four years and did all the worksheets."
Besides fitness-related fields, being in a sport might simply demonstrate that a student is hard-working and stands out from candidates who go home and watch TV after school.
Debate is a little different, because it directly teaches research and public speaking skills that are unmatched by classroom instruction. Any flaw in your choice of references, or interpretation of those references, will be torn apart by the opposition. That's their entire task for the whole 90 minutes, so they're going to find flaws even where there aren't flaws, and your job is to defend your position against that. It's more scrutiny than you get from a teacher or professor, because the point is to make you lose. It requires more preparation than writing one paper, because you don't even know what arguments you might encounter, and therefore must have an assortment of supporting research, and be familiar with it enough to find it during an opponent's speech. And you have to be able to defend and rebut your positions quickly and articulately, because they are asked in real time in front of a judge. I can't tell you how useful that practice was to me in college, and to my teammates who went on to do much more than I did.
Getting into a college is one thing, but being prepared to excel in it once you do is also important, and that's really the point in high school.
