If you've just booted up your Raspberry Pi, but don't have it connected to a screen and keyboard, you probably want to log in from a Terminal window, but don't know its IP address. Pi Finder figures it out for you, and makes it easy to connect. Pi Finder also shows you the MAC (Ethernet) address of your Raspberry Pi, so you can add it to your router as a DHCP reservation, meaning your Raspberry Pi will get the same IP address every time and you won't need to run Pi Finder again.
