Let's say my dashboard has two panels with timechart displays. As I hover over panel1, I'd like for panel2 to show an indication (possibly via a vertical line) of the time in panel1 where my sprite is currently. This makes it easier to correlate across multiple panels, and can help when analyzing the charts.
The Analytics Workspace does this by default. Note, this was called Metrics Workspace prior to 8.0, and was not installed by default prior to 7.3.
Current docs: https://docs.splunk.com/Documentation/Splunk/8.0.2/Analytics/About
For 7.1, 7.2: https://splunkbase.splunk.com/app/4192/