Timesheets are particularly useful for people that choose to enter their time at the end of the day or week (although they can also be used in a more granular way if chosen).
Each timesheet row gives you the option to select a Project, Work Item, and Client. However, none of these are required. If desired, you can log time against nothing. This may be desirable if you want, for example, to keep track of time spent in meetings. You also have the option to log time against a work role and team. Selecting these options can be useful if you want to run reports against a particular work role or team.