The concept of “end date” is the one of a softer version of “due dates”. While “due dates” have a specific deadline that should not be missed, “end dates” implies meeting a specific deadline is just a nice to have.
The advantage is that it allows an automatic sorting of tasks by degree or urgency:
Tasks with (approaching) due dates can be focused on first
Tasks with (approaching) end dates may be selected as next in line,
etc.
Hence, it would add an interesting degree of granularity between tasks which have a due dates and the the ones that do not.