Face ID lets you carry out authentication without having to interact directly with your device. For example, say I want to enter a password into a safari webpage. With Touch-ID, I need to rest my finger on the fingerprint sensor. With Face ID, I just stand there looking at my screen and it gets done automatically.
This essentially saves you a step.
So what this means is that unlocking my iPad Pro is as simple as hitting spacebar on my Smart Keyboard twice. It also allows me to use Siri from the lock screen without having to unlock my device as well, since it automatically unlocks when I am looking at it.
Under the right situations, this means lesser friction when interacting with your device.
I could name a couple more, but I think you get the point.
Yes, what you write is correct. But for me personally this one "saved" interaction does not compensate the disadvantages I get with Face ID. How much time does it take to put my finger on the sensor? Is this one move really too much effort? Can't think so.
Without Face ID and Touch ID under glass instead, the front of the phone would only need a small notch on top of the phone for the front camera (I would place it centered on top). This design would come very close to the "perfect" phone with only a display on the front. And then, please, not more than two cameras on the back, the fastest SoC available on the market inside and I would buy it right away!