If I look at my daily agenda I want to see my schedule for today: hobbie, work, family, sports. It's not comfortable to see how my tasks overlap each other - there are a lot of.
So if I able to create expandable group event and put all my events inside, then I avoid mess in agenda. And from other side all events inside the grouping event would be limited by parent event time. It's cool when I'm planning a birthday party or business meeting. All stuff is hidden inside. And I can instantly focus on "current time" tasks.
To represent this you could create thin colored border for grouped events or background with transparency for a grouping event (parent). Currently I create one (parent) event to see my schedule and time limits that repeats daily and other events that should be fit in this timerange of grouping event.
It's very uncomfortable to see how much space the parent task takes.