It allows some more flexibility. For example, with blackberry's default calendar you just have show tasks or not.
With PI you can show tasks in the week view, but not the day view.
I also like that it's all the functions in one application so I can work on tasks and then press m to go to the month view (all keyboard shortcuts are configurable too).
I like that I can color categorize things and use icons. I can tell by a glance who has what scheduled without having to take the time to read each entry.
Posted from my CrackBerry at wapforums.crackberry.com