Reel SEO wrote a great article on the use of annotations. They suggest that using annotations are an efficient way to boost your videos’ performance. Here’s why:

1.  Annotations keep your viewers engaged longer, and Google picks this up, so adding annotations enhances your SEO.

2. You can use annotations to direct people to your other videos rather than have random videos appear at the end of your video. This gets more of your content seen.

3. Using annotations with arrows and directions helps users to navigate easier. For example, you can use an annotation to point to your subscribe button.

4. Annotations are a good way to update older videos by posting an explanation.

Here’s the link to their article

http://www.reelseo.com/how-to-annotations/