It is a tool to remove unwanted Apps from running at Login. After turn on the Mac, various apps, add-ons and many invisible background APPs open by themselves. Too many items cause slow MAC and low battery life. Many times some of these are visible and some only show in Activity Monitor. You may not recall how did this APP started and what may be logging your Mac.
Startup Manager will show all the possible APPs that runs on Login. It is safe and lists only non Apple APPs. It keeps track on the items that are automatically started; either during startup, during login, or during another applications launch (e.g. browser extensions).