What's the deal with the apps that register themselves in the Mac menu bar uninvited?

Most already have a full-sized Dock icon with a dot underneath to indicate that they're running. Why do they have to be somewhere else? These menus are often useless:

Pumble's menu bar item whose only option is 'Exit Pumble'

Sorry buddies, it's either the menu bar or the Dock. You can't be in both. Good thing macOS lets you turn these little buggers off in System Settings > Menu Bar > Allow in the Menu Bar:

macOS 'Allow in the Menu Bar' settings with menu bar items turned off for most apps