I'm afraid I don't know what you mean by "roll ups" - are you referring to summary tasks?

By default subtask data (work, cost) values to "roll up" to summary level tasks. The advantage of summary tasks is organizational. You can create "Phases" in your project and see the cost per phase or the work per phase.

There is a setting in Task Information to "roll up" task bars to summary - that is purely visual and causes and subtask bar to appear superimposed on the summary task bar.