For a possible thing, existence and nonexistence, or realization at different times, are conceivable alternatives. Simply saying “it is possible” does not explain why one determinate outcome occurs; this is the issue of preference and specification.
If a possible state is said to assign its own definite time merely by being possible, one has added a determining power that possibility alone does not supply. Will articulates the specification of one possibility.
Key distinction: “having power over an outcome” concerns its possibility; “its occurrence at this particular moment” concerns its specification.