For some cases, we would like the ability to set the "Do Later" based not on a date, per se, but on a Milestone. For example, it might be a particular release (for post-release tasks, etc.).
Then, if the Milestone date changes, the Do Later date should update as well.
It would also be quite nice if you could set based on a Milestone that has no date set, and the Do Later inherits the date when it does get set.