Films are rarely shot in sequence and the filming of them are often split by location or cast availability, etc.
The call sheet (schedule) has all your filming commitments on so you can plan when you're needed and which scenes you will be performing on particular days so you can make sure you have the appropriate lines learnt.
The call sheet will have been carefully planned taking into account everyone's availability. Sometimes locations and equipment have also been booked and paid for, for a particular day. It is vital that we follow the schedule as re-organising it can often be extremely complicated. Having to change plans can also put the production at risk.