When reviewing the Google Analytics over-time graph and screening website traffic spikes on a daily, weekly or monthly basis, the lines and graphs will not explain to you why the traffic is moving in either direction. For example, today you might have had 6,000 site views, but tomorrow you might receive over 10,000 site views. Tomorrow will be the day that your new marketing goes into effect and an email blast will be sent out driving traffic to your new monthly promotion. This is a perfect example of where Annotations would be used.
Any user with access to your Google Analytics profile can leave notes right on the
over-time graph. Instead of looking at a raw set of data and numbers an Annotation will give you a better sense of why your traffic increased or decreased.
These notes can be as simple or as detailed as you make them. Annotations can become your sticky notes for all information pertaining to your marketing endeavors or website issues. To create an Annotation, you can either click on the day you want to annotate or click on the down arrow on the bottom of the over-time graph and click on on the right side of the tool bar.
For instance, your web server might have gone down for six hours; it would be to your benefit to make an Annotation to that fact. Otherwise you would see a downward traffic spike and not know the root cause.
Beyond marking traffic spikes, Annotations can be used to mark changes to your website design or layout, mark the start and end of any campaign or notes or change of keywords.