I do a lot of macOS installations and trouble shootings and there are many cases that requires for booting from another bootable device. It’s not a big deal because you can easily hold down the option (alt) key to activate Mac’s built-in Boot Manager and choose the disk or the volume that you want to boot your Mac from right?
Well when you use an old Wireless Apple Magic Keyboard that only connects via bluetooth or you have to leave the Mac to do something and reboot when it’s done you might miss the chance to hold down the option ket for Boot Manager to show up.
And lately I’ve been using OpenCore Legacy Patcher to install macOS on unsupported Macs but sometimes when the Mac reboots it can’t find the USB disk that I’m trying to boot from and install macOS with so I have to turn the mac off and then turn it on again and hold down the option key to resume booting from the USB disk.
Enable Boot manager app will turn the Boot Manager on by default so every time you turn your Mac on or whenever it reboots you can choose which disk or volume to boot from.
This option could save you time and some unnecessary headaches.