In IC design, what is the Buffer for ?
Fundamentally, a buffer is an amplifier. It takes a small signal (lightly loading the source of the signal) and provides a copy of that signal that can drive a heavy (e.g., capacitive) load.

They are used in places where connecting the heavy load directly to the source would adversely affect the signal. Such effects arise because the signal source has a nonzero ouptut impedance, and the output may also used for feedback (or to feed other loads) in some way. Loading the output also affects the feedback, which then changes the behavior of the source in undesirable ways.