Roster is commonly referred to as your work schedule. The roster is more commonly used in companies which have 24-hour work shifts and there is a constant need to rotate employees to and from various work shifts.
An example would be an airport which runs 24 hours a day, you will have to assign employees to different shifts on different weeks or months, you will not assign an employee to a night shift from 0100 am to 1000 am throughout the employment period. You will need a system which does this efficiently.
Rosters are most commonly used in workplaces where there is a need to rotate employees around various locations or timings or other work-based scenarios, like restaurants, oil rigs, airports, pilots and other such scenarios.