Media posted on the Web should be made accessible to individuals with disabilities, including visually and hearing impaired people. This means providing transcripts and/or captions for audio and video. Providing captions for media is a broadly accepted accessibility standard, as reflected in WCAG 2.0 as well as Section 508 of the Rehabilitation Act.
You can understand the need for transcripts and captions if you keep in mind how people may access content. Blind individuals often use screen readers, which is software that uses audio output to interpret and "read" the content, such as transcripts, out loud to the user. Hearing impaired individuals may depend on captions and transcripts so they can access audio by reading. Some situations require only transcripts. Others require captions. Note that to make captions you must first have a transcript.
Special thanks to Dave Giberson of the San Diego Community College District Online Learning Pathways group for permission to borrow from his excellent guidelines and tutorials.
Quick reference of guidelines for creating transcripts and captions.
UCSF videos about how to make media accessible
Tools and best practices for adding captions to video.