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...
The thing is, though, for many business models being able to accurately estimate time and cost is critical, and so is executing vs. that estimate.
It's nice to be able to say "I have no idea how long this will take," but it's also a luxury not available to many (most?) developers.