While a buffer between sequential filters does allow us to multiply their transfer functions, in fact a buffer's use is broader-- the idea is to ensure that circuit elements don't affect each other.
If you don't have a buffer, you will always get a voltage drop over a load, because the circuit acts as a voltage divider when including the source impedance. You might not want that if you want the output voltage to follow the input voltage. A unity-gain buffer ``protects'' the load from the source impedance. A perfect such buffer will have infinite input resistance and draw no current from the source. In general buffers ``isolate'' input circuits from output circuits, so they don't affect each others' impedances (and they are very common in circuit design... we'll see examples).