It's easier to reason about highly asynchronous code if all you can do is transform and compose streams, and all side-effecting work happens at the "ends of the pipe", so to say.

Arbitrary side-effects coupled with asynchronicity are a recipe for Heisenbugs.

Additionally, if you add in the constraint that you can't change signal routing at runtime and you only use pure functions on stream, you can actually debug by moving streams to any point in their history and figuring out what the system is doing with them. That's a very common debugging technique in the Elm programming language, for instance, as shown here: https://www.youtube.com/watch?v=RUeLd7T7Xi4.