Transcripts serve several groups at once: deaf and hard-of-hearing users who cannot hear the audio, users in sound-sensitive or noisy environments, people who prefer reading to listening or watching, and search engines, which can index transcript text but cannot index spoken audio directly.
Unlike captions, which are synchronized and displayed over the video itself, a transcript is a standalone document, typically linked or displayed below the media player, and WCAG accepts a text transcript as an alternative for some prerecorded audio-only content under success criterion 1.2.1.