TMTA / Transparent

I have never understood transparencies. Why use them? Why not stamp directly onto the picture? Could someone please explain!
Anyway, I found myself extremely challenged when TMTA was prodding to use them on this week's art.
This is the result:

Enter in Paris.
On technique:
Black stamping on white paper, blue stamping on a transparency, both glued on top of each other and then on gold (some of it peeking as sky), a 50's lady and a piece of foil (from a dog food package, peeled off real easy!) - voila!

I'm so happy to be challenged! Otherwise I would just stick in my old rut. A huge Thank-You to the folks at TMTA!