Most are from very respected people to help others benefit from their knowledge and to avoid the pitfalls they ran into a various levels of scale. Do you need B

Most are from very respected people to help others benefit from their knowledge and to avoid the pitfalls they ran into a various levels of scale. Do you need BEM for a brochure-ware site, no, styling by IDs and tags would probably work just fine. But a web app with 1000s of lines of code, while I don't specifically endorse BEM, some sort of pattern and methodology will save you a world of pain in the long run.