Large backlogs make more sense in groups. The purists say that the backlog should be a single prioritised list. Well, if you're a purist, you can have a single prioritised list.
But if your backlog is 500 items long, it might be easier to understand what is in the backlog, and to prioritise it effectively, by grouping backlog items under common themes. OnTheSamePage lets you create as many backlog groups / themes as you want and organise them in whatever way you see fit.