Exactly! The subject tests are a way to signal an admissions officer that you are a strong student who is an expert in a certain field. It can be a great way to complement your overall story (your personal statement, your major) or even “distract a little” from your SAT performance. Don’t get me wrong: your SAT score will still be looked at first. But if you want to major in Spanish and score 800 on the subject test but only 500 in Math and 700 in English, the AO would not weigh your Math performance as heavily. SAT subject scores are just one more way to stand out (if you perform well).
In terms of necessity: it depends on the schools your want to apply to. I know that some schools (e.g. Georgetown) strongly encourage you to take at least 2. Most schools however state that it is optional. Given the continuously dropping acceptance rates of top schools you are probably a more competitive applicant if you do have subject test scores. Bottom line: You should only take the subject tests if you can score a near perfect score because the students who take these tests are usually experts in the field. If you don’t think you would score too well you can always make up for it with extra curricular or — the most important part: your personal statement. I hope this helps! Let me know if you have further questions and do not hesitated to pm me! 
good luck