I assume there is a folder that contains all event sheets, but the layouts folder also show which event sheets are associated with which layouts. It may not be clear in the screenshot, since it is a simple project with only one layout, one event sheet, etc.
I would assume (again) that if you had several event sheets associated with a layout, and event sheets used multiple times across several layouts, the project panel would show how everything is related.
What I'm wondering is, why Object Folders? Why not just 'Objects', then have sub-folders inside it?