We've already seen how we can handle an error in the observer. However, by that time, we are practically outside of the monad. There can be many kinds of errors and not every error is worth pushing all the way to the top. In standard Java, you can catch an exception at any level and decide if you want to handle it there or throw it further. Similarly in Rx, you can define behaviour based on errors without terminating the observable and forcing the observer to deal with everything.