Different things. A pre-ferment (poolish, biga) is for developing some of the more complex flavors that a quick / "straight" dough won't give you.

Tangzhong is basically about packing more moisture into a dough while keeping it easy to handle. Higher hydration doughs are very sticky and take some experience to work with, but this will let you get something that is like a 75% hydration, but still handles like a 70% hydration dough. (I just made up these numbers, but you get the point).

For my part, I am fairly experienced with handling up to 80-ish% hydration, so I never bother with tangzhong, since it just adds another thing to do and creates more dishes to wash.