If you look carefully at the schematic for this Project, you'll notice that it looks like an amplifier circuit. It turns out that's what an RTL buffer is. After building this circuit, set the Power Switch to ON.
Now press the Key. You'll see LED 1 light. The input is 1, the output is also 1. So why do we need circuits like an RTL buffer?
Many times we want to control the operation of some device, such as a lamp, that requires more current than a digital circuit can deliver.
A buffer circuit between the digital circuit and the external device Lets us amplify the digital output enough to "drive" (that's engineering talk meaning "to operate" or "control") the external device. Strictly speaking, this buffer circuit isn't a true digital circuit. But buffers are important in digital electronics to help digital circuits get along with the "outside world".