If you have not read Indexes 102 Jump There Now then read this.
Trends in Unified Logging provide a way to jump to points in time for specifics index points of interest. By using Data Indexes you can look at areas of your application or specific servers for a day by click on a data point then dive deeper into an hourly breakdown. Once in the hourly breakdown you can click on an hour and be taken to the messages for that time frame.
The image shows an Index set for a user and the breakdown by day then hour. Trends will show all indexes not just data indexes but using data indexes helps segment the data into a usable view.
Please note that the trends page works in UTC time and the messages page translates this time to your browser time.