YouTube indexes its videos from the contents it identifies. This is: the video title, a brief description and tags, and people comments.
Other than your title and brief description, you need subtitles to help YouTube properly index your video.
Video transcription helps YouTube to index your video contents in your language, so:
The same goes for your video subtitles in other languages.
Add subtitles to your videos and you'll help people from many more countries reach your videos, and fully understand and enjoy them.
Subtitles and transcription help your video to appear in lot more YouTube search results, both in your language and others.