What are 'annotations' used for in Google Analytics?

Annotations in Google Analytics serve the critical function of allowing users to add notes directly onto their data timeline. This enables analysts and marketers to explain changes in traffic trends or other noteworthy events that may impact the data, such as marketing campaigns, website updates, or external factors. By adding these contextual notes, users can easily reference and analyze how specific actions correspond with changes in user behavior, making it easier to draw correlations and insights from the data over time.

The other options, while relevant to various aspects of data analysis, do not accurately define the purpose of annotations. They do not capture the specific function of adding explanatory notes to facilitate better understanding of data trends within the context of the timeline.