How else would one forecast?
If you need to accomplish TaskX, you work with the team that will do TaskX to come up with a valid estimate of effort required to complete it -- say, 300 hours of work.
You could just naively assume that this task would then be done in 300 hours / 8 hours per day / # of workers, but that assumes 100% productivity -- which isn't true basically anywhere.
Ergo...