The core requirements are closed captions for video and a transcript for audio-only content, both of which WCAG requires at specific conformance levels. Some deaf users, particularly those for whom sign language is a first language, may also find plain, clearly structured written text easier to process than dense paragraphs, since English or another spoken-language script functions as a second language for them.
Auto-generated captions are a useful starting point but frequently contain enough errors, especially with technical vocabulary, names, and accents, that reviewing and correcting them before publishing is genuinely necessary rather than optional.