Employees can each have different rights to see things and perform certain operations.
Each employee has their own user rights. Those rights affect how they use the the timesheet and which features they have available to them. You can set rights for an individual employee or an entire workgroup. When you set rights for a workgroup, it affects all the users under it.
You can set rights so an employee only sees the timesheet and nothing else. Or, you can give them access to project tasks, time logs, expenses, invoicing, and time off. Each user can have their own settings. Even if an employee has access to project tasks, you can lock them down so they cannot create new project tasks, modify them, or delete them.
Administrative rights are in addition to the basic user rights. You can lock down "lower" admins so they cannot administer users, or projects, or clients, or a host of other activities.
Most users will never miss features that have been hidden from them. They'll simply enter time into the timesheet and never even think about other aspects of the program. And in some cases like expenses, invoicing, and time off, they may not even be required to use those features. Eliminating features they don't use simplifies the program and reduces the number of menus and tabs they see. That reduces frustration when hunting for features they do use.