There are a few reasons to put a "manual" activity item into the gradebook.
One is because the unit might have activities that exist outside of LEO, such as a presentation, or a simulation assessment. If nothing is submitted through LEO it may be useful to create a manual item, so that the lecturer can still place the grade/mark and any comments into the LEO gradebook directly.
Another reason to create a manual item, is to hide the raw mark for a Turnitin assignment. A Turnitin assignment is only capable of displaying a numeric mark. While this mark can be displayed as a grade in gradebook, a student will still see the mark if they go to the Turnitin assignment directly. The workaround is to NOT put the mark into Turnitin, but instead put the mark/grade directly into a manual item in the gradebook. Comments and rubrics can still be used in Turnitin, but no mark should be attributed to those comments in Turnitin itself.
From Grades > Setup > Categories and items, click the 'Add grade item' button near the bottom of the page.
Give the item a 'Name', a 'Maximum grade', select a 'Grade category', if relevant, and change any other settings as necessary.
Click the 'Save changes' button.
Move the item to the relevant position in the gradebook and save the changes.