The scheduler window lets you do one thing quickly, which
is set the start, do or due dates for an action. Basically,
you use it by selecting a collection of items in the Action
Browser and bringing up a Scheduler window. From there you
can select any or all of those items and quickly go through
and set their associated dates.
Note that Actions aren't considered actionable before their
start dates so you should think of the start date as the
date you have start thinking about the associated actions,
which may be before you can actually do the action. Or
perhaps you should make two actions: one for planning for
the action with an early start date, and one for doing the
action much later.
Note that you can also schedule via command-T for today,
command-option-T for tomorrow, or the number keys command-1
through command-7 for Mon-Sun command-option-1 through
command-option-7 for next Mon-Sun.