Adding subtitles to your videos is always a great idea. They can let the viewers watch it without sound (which is important for the silent autoplay, that is making it's way to the new versions of iOS and Android), make it easier to understand what has been said if the audio is too noisy or lower quality, and help non-native speakers better understand the spoken word. It's also necessary for people with hearing impairment.
YouTube understands that and they encourage the content creators to add subtitles to their videos. They offer a great way to easily create transcripts, which is usually a time-consuming process, or requires a specific and expensive software. It offers two options: creating an automatic transcript and putting in your own text and have it synced automatically with the video.