I think it's this whole notion of being a well-rounded individual. In many other countries, you choose when you start high school what path you want to take and you don't have a broad-based education. I think there are pluses and minuses to each approach.

if you're good at chemistry, it shows a college that you can think theoretically and then apply those theories in real world situations. this is a step beyond just understanding an equation.

performing chemical reactions and then computing the results and drawing conclusions shows that you can both perform complex computations, and then explain what those computations mean.

I think soccerbabe hit the nail on the head here.

College is all about critical thinking, and drawing conclusions from experiments shows that you can think critically about the work you do and what it means.