Not a matter or being enough people with hearing impairments, though that is about 10% of the population, and about 40% of us, who will lose hearing significantly as we age, and many foreign language speakers who use captions to help them learn and understand English, and many hearing people in families with hearing impaired people, and many people who the captions help when people on screen are whispering or mumbling ... let's see that brings us to almost everybody can benefit from captions from time-to-time.
More importantly, it is required by law, to accommodate those with hearing impairments.
Differences between closed captions (CC), subtitles, and Subtitles for the Deaf and Hearing impaired (SDH)
in general, closed captions and subtitles are designed for deaf and hearing impaired people to replace not just language, but also sounds (doorbells, gunshots, explosions, rustling in the bushes, etc.).
Subtitles in general only replace language.
Closed Captioning has been around for a very long time, and most broadcast programs are required to provide Closed Captions. Televisions (over 13"), since 1993, sold in the United States are required to provide a decoder for Closed Captions for programs broadcast over the airwaves (antenna).
Recorded media, like VHS and DVD generally contained Closed Captions, and the television would decode it.
The advent of the HDMI cable, which does not carry the closed caption embedded in the composite signal changed the way many televisions dealt with captioning. On most older televisions (1990s-early 2000s), if you played a DVD through any of the inputs on the television, the television would decode that closed captioning information from that DVD (or VHS). Recent televisions moved the closed caption decoder so it ONLY decodes signals from the antenna (broadcast), despite that there is no additional cost to design and manufacture the decoder downstream so it could decode signals from component, composite, and the antenna - I guess they assumed everyone would be using HDMI and subtitles for the hearing impaired (which means they assume we are going to throw out all of our old DVDs - my uncle still has some VHS tapes even).
Anyway, since HDMI does not transmit the encoded closed caption information and most newer televisions don't decode video sources (only broadcast sources), and many new televisions only have HDMI inputs for our a/v devices, that means that they only way to get the closed captions from our older DVDs to the new televisions is to decode them before sending them up the HDMI cable, so they become part of the picture/video (as someone else said, "open captions" or un-encoded).
Is this issue important? It is to those of us who live with, visit, invite, or socialize with deaf or hearing impaired people and would like to enjoy watching movies together. It should be important to hearing people, since a good number of you will suffer significant hearing loss as you/we age. It should be important to many foreign speaking immigrants, and to those of you who keep complaining that immigrants should learn the "American" language.
Why some people dismiss this as unimportant because it only serves a minority (a minority who cannot perceive sound), I can not begin to fathom the shallowness of those minds. It should be important for everyone who has any sense of community and fairness!