I’m a semifinalist this year. I got National Merit, and the rest of my admissions profile is pretty competitive (11+ APs, 34 ACT, regional programming challenge champion). Would this help my admissions a ton?
For many colleges yes. Particularly these: http://nmfscholarships.yolasite.com/ For highly competitive schools, not really. They are looking at your entire record.
For selective schools, being national merit semifinalist does not increase your chances much as tons of people get that award.
However, being National Merit can help you get a good amount of scholarships at that school. For example, USC gives National Merit half-tuition.