Some events have known expected duration, but flexible start dates (and thus no hard deadline, or rather a moveable one), more like a lead time to completion than start/end dates. This is also not the same as time estimate, which estimate the number of hours needed to be allocated for work completion.

It would be good for processes that have a known or even agreed SLA lead times, to have that field in a task.

this is similar to the request for start&end for Gantt