I feel like this is is high school lit level advice. I've heard that in the US, part of the grade on essays is calculated algorithmically, based on the rarity o

I feel like this is is high school lit level advice. I've heard that in the US, part of the grade on essays is calculated algorithmically, based on the rarity of words one uses. My teachers also tried to get me to avoid word repetition in my writing, such as the word 'very' 'say' etc. but never explained to me why it is a bad thing. My personal theory is the usage synonyms is important to make sentences look 'dissimilar' as possible. This aids in reading comprehension, as samey looking sentences can lead the readers to lose track of their progress, similarly how one can lose their way physically without the aid of distinctive landmarks.