But what is the application of Code Editor in Rules Editor? When do we use it?
In those cases, how do we ask Marketing people to code?
Code Editor was built for a developer persona to write complex rules. It is never meant to be used by a marketing guy. In fact, for 6.3, we have added another layer of security for code editor to be usable only for a particular set of users and not all.
If you have a complex rule in your form which is to be re-used, you could create a fragment and pre-bake the rules in the fragment. This way, the marketing guy just has to drop the fragment with rules pre-configured.