Buffer Purpose?

[MattAnonymous]

What exactly does a buffer do and why is it needed?

[Transmogrifox]

Short answer:

Isolate output impedance of previous circuit, provide extra current to drive following stage at input voltage level.

It just keeps the circuit from interacting with whatever comes before or after it, since who in hell knows what is going to be plugged into the input, or what someone will put on the output.