Why not? Not everyone is in a room by themselves and can turn up the volume. Not everyone can wear big gamer headphones at work.
Posting a video on the internet and not including a transcript is like posting something on Instagram and no other platform and thinking, "That's good enough."
If your goal is to communicate your thoughts, you make them available to as many people as is practical.
You don't publish a book and slip it onto one shelf at one local library and then wonder why nobody reads your work.
Yes. I turn CC on for most videos, even movies or shows watched at home.