Unbuffered vs. Buffered CMOS Circuits
It's to do with providing stable and predicable output drive characteristics for a CMOS logic device.

Read TI's document on the subject

Buffered CMOS
A buffered CMOS device is one for which the output ON impedance is independent of any and all valid input logic conditions, both preceding and present, and is said to have a buffered output or to be a buffered CMOS device.