There is a finite number of freshman spots available and high score subject tests will give an advantage over another applicant without them. Pretty simple concept.
For example, many of the UCs only require 2 years of a language and 3 years reccomebded. However, to be a highly competitive applicant, many kids are taking a 4th year of a language. You can bet that all things equal the kid wth 4 years of a language versus a kid with only 2 years will have an advantage.