Hi Maria, 

I know question wasn't pointed out to me, but I can give you one use case:

Say you have a project with a lot of tasks (smaller tasks that last 2-3 days) divided in phases (10-20 of them), and this project lasts for 6 months or year.

This means you have a lot of items in gantt, which can scroll deep (say a couple of hundred of tasks).

Sometimes you just need an overview of the project phases, and if you collapse to phases, then you don't see whats within them. If you don't collapse, then you have a lot of scrolling and this is not an overview anymore.

But if you can have this kind of "overview" view, then you can see it with minimum scrolling and have better overview. Then if you need action on some item in phase, you expand this, do necessary change and thats it.

I hope its sufficient explanation. If you need any additional info, please feel free to write.

Best regards,

Vedad