You'll use it to unlock your phone and pay for stuff
Much like Touch ID works for iPhones with a Home button, Face ID will function as the default security system for the iPhone X, meaning your face serves as your password instead of your fingerprint (your passcode will continue to be your backup method of unlocking your device). It will also function as a secure way to confirm transactions using Apple Pay, and for third-party apps (banking apps, financial services apps, etc.) to verify your identity.
Essentially, anything you would have previously used Touch ID for will be replaced with a quick scan of your face.