If you are used to being able to select which indexes you want to see, like on the Trends page, then this will feel very familiar. Now when on the Messages page you can select which message matching which indexes you would like to see.
Why is this important?
The screen shot shows the demo account and the indexes it contains. So if you only want to see messages for the color #11AFB1 you are able to choose the index for that color.
Another example would be creating an index for each server/computer then you would be able to filter down to only messages from particular servers.
This new feature removes some of the trees from the forest so you can see the cabin.