I've noticed that some generating stations have "generation schedules" and similar schedules also exist for load distribution stations. They usually have a MW amount telemetered to the ISO (grid operator) for each operating hour. How do they determine these values in their schedules knowing that they could change throughout the day? What significance do they have?
The fact that demand might change during the day doesn't mean that the grid operator can throw their hands in the air in exasperation. Planning is still required and deviation from plan is to be expected if the situation changes.
Planning will be based on historical data, season, weather forecasts, large user demand planning information, the famous big TV event ad-breaks, etc., along with other generators' production schedules (solar and tidal are intermittent but predictable) and maintenance schedules.
This will lead to a plan with adequate base generation capacity and, if possible, some generation that can be started quickly - hydro, pumped storage, gas turbine and now battery storage. Don't forget that the system should be able to sustain the loss of a generator due to a fault.
What significance do they have?
They're "a plan to deviate from".