The dialog "Several tools require Parallels Toolbox to be open to function. Are you sure you want to quit Parallels Toolbox? If you Quit Parallels Toolbox, some tools will not continue to function or send notifications." So exactly what tools do I need (that I'm not aware of) when I decide to quit Parallels Toolbox?
mostly Time-related tools (timer, alarm, etc) and state-changing tools (Do Not Sleep, Do Not Disturb, etc). They require main Toolbox app to run.