Why do you need to know vocabulary for the SAT?

I’m trying to devise a study plan for myself for the SAT, and I was wondering why exactly do we need to know all the vocabulary words that everyone stresses over and that test prep companies place so much emphasis on? I was thinking it’s mostly for the sentence completion questions and some of the passage reading and comprehension sections, but honestly it doesn’t seem like a lot.

I don’t know what to say for your first q besides the fact that that’s just what the SAT tests, because that’s what CollegeBoard agreed on, but this isn’t the same for the new SAT. I believe they’re taking out the sentence competition for that, but leaving in the vocab in context. I recommend the Barron’s and the Barron’s 2400 lists as well as the words from Direct Hits List books. These websites: http://d1w9n6kt3pr4mi.■■■■■■■■■■■■■■/SAT%20VOCAB%20LIST%20v%202.1.pdf and
http://vividvocab.com/home/book?
have also really helped me a lot too. Honestly, learning 10 words ish a day through visual sentences (see latter website) really helps me a lot, and in the practice tests I never miss any problems in the sentence completion section. Of course, sometimes, you may not be able to memorize every single word, but you can still use process of elimination after thinking about what answer you can fill the blank in with.
And that’s good for you if you say it doesn’t seem like a lot. That probably means you already know a lot of the most common words, but devise your study plan or the amount of study time you want to put into the vocab words depending on how much you know and what you want to get.

If you’re prepping for the revised 2016 SAT, you will not need to know so much esoteric vocab.

If you’re prepping for a test between now and March 2016, though, you need vocab because they test it. There are 19 sentcoms, all of which test vocab explicitly, after all.