Proper rotations came from parsing. All these guides you read, all these video guides you watch, all these threads or posts on how to DPS, all of them have a root in parsing. Sure trial and error as well as experience come into play with these guides, but parsing is just as valuable as the aforementioned, since knowing when your abilities will do the highest amount of DPS is key against enrage/DPS checks. Also you mentioned theorycrafting earlier, which is all about numbers... which a parser is all about.