It has been a very clever tool to manage staff holidays internationally. It takes away the admin burden of management time having to keep things up to date and puts the empowerment to the individual to manage their own holidays as they can also see when their fellow team members are away. It prevents wasting management time and avoids disappointed staff as they know already whether their holiday is likely to be accepted.
I also like the sickness management facility and have found it useful for managing staff illness.