Tasks are often dependent on each other.
- I can't start editing until Bob finishes writing.
- We can't put the roof on until all four walls (each with their own projects) are completed.
- Our sprint tasks all start at the same time.
- My oversight starts when the list of the contractors' work starts and ends when all of their work is completed.
So it's great to make pretty GANTT and Calendar pictures, but we need to model the work dependencies if we're to leave other tools.