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..not because they are supposed to be accurate or they are 'right'.
But because, a schedule can:
1. help track your work progress. Breaking a big piece of humongous work into small chunks and tracking against time.
2. help encourage accountability, commitment and frequent communication.
3. tie people together in a project and help them being part of the process.
It should not matter if your schedule is inaccurate or your project progress is not respecting your schedule.
Though, it does not mean it should not strive to make it as accurate as possible.
Try your best with this but it's more important to make sure you have a schedule.