The ability for YouTube to automatically generate captions for hearing impaired users was intended to open videos to an even wider audience.
Unfortunately before now, the automatically generated words would often end up being completely incorrect or even embarrassing. While possible to add your own captions, you would have to create a captions file and upload it to your video, a process which could be very time consuming.
Fortunately, captions can now be edited directly under the captions tab on YouTube. Now if you have just one line of text that doesn't make sense, you can fix it without having to create an entire caption file from scratch.
There hasn't been much talk about this feature (that I know of), but it should be live for all users.