Annotations explain how wireframes work

Annotations provide the information that static wireframes just can't. Usually, they detail how something should work on interaction. A developer, of course, recognizes that a button is interactive. But without an explanation, they wouldn't know what to make that button do. Same with copy and UX writers — they need to know the voice for the wireframe text and what information the copy should contain.

The person who creates the wireframe might experience a sort of "expert bias." You know so much about the wireframe that you might assume that everyone who looks at it will clearly understand your ideas. However, others don't have the prior knowledge that you do, and their assumptions about what something does are just assumptions. To help others, add clear information about what you want each element to do.