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.