They compose. And can be passed around and be completely oblivious to how they will be reduced. With conj or sum or whatever they want. And you can extend them at any point at any end.

They are like map, filter and friends, but they compose. I think of iterators as an iterator protocol and transducers as a streaming protocol. An iterator just describes how to iterate over a collection. Transducers are transformations that can be plugged into any point where data goes in one direction.