Good questions. In general, you (probably) need an RNN any time you could benefit from long term dependencies, and/or want to handle sequences that very a lot in length. Currently, RNNs are the state of the art in speech recognition and language translation. Both of these tasks require generating a sequence, which is something I haven't seen done effectively with CNNs.
For those that like sausages, you may like to try other flavors. However, at this stage, few people are making good sausages, so in many situations the flavor you want isn't available - so for now, at least, you'll often find you need to know something about how to make them, to get the kind you need.
Also, it helps to understand how they're made, in order to make your own, better, sausages (which you'll often want to do, whilst the existing menu is so sparse and of sub-optimal quality!)
PS: I changed the thread title so I can start the lesson discussion with links to the resources - hope that's OK...