Channels are many-to-many; many goroutines can write to it simultaneously, as well as read from it. This library is pitched right at where people are using this, so it's rather a key feature. An iterator is not. Even if you wrap an iterator around a channel you're still losing features that channels have, such as, channels can also participate in select calls, which has varying uses on the read and write sides, but are both useful for a variety of use cases that iterators are not generally used for.

They may not be "necessary" for concurrency but given the various primitives available they're the clear choice in this situation. They do everything an iterator does, plus some stuff. The only disadvantage is that their operations are relatively expensive, but as, again, we are already in a concurrency context, that is already a consideration in play.