This flag background allow to keep autosave on when modal operator is running.
Can solve probleme like described here : https://devtalk.blender.org/t/how-to-read-mouse-keyboard-continuously/2212/5
Having this distinction seems reasonable, however there would be more control if this is an argument to WM_event_add_modal_handler (and the Python wrapper).
Then the flag can be stored in the operator wmOperator.flag.
Note that I assume this will be used for other things besides auto-save in the future.