It is not possible to enter time only against activities in Project and Billing edition of Web TimeSheet. However, this is possible only if you have Time and Attendance module provided if Allow time to be entered option is selected to Both or Without requiring a project to be selected under Timesheet section in the user's permissions.

In Project and Billing module Activities are common functions that are performed across multiple projects. For example, a consulting company has three core functions – budgeting, design, and testing – that must be performed for every project they complete. Instead of adding these to each project as tasks, the functions can be defined globally as activities.