Because sometimes the native AE tools don’t offer as much flexibility as the pricy 3rd party stuff. If a client came to you and wanted a specific effect like particle trails or something of that nature. Which plugin would you use? Trapcode Particular or CC Particle World? (remember Cycore Effects weren’t always bundled with After Effects. In fact it was a 3rd party plugin itself up until a few years ago. Also, it’s a variant of BorisFX Final Effects Complete collection). Both have their strengths and weaknesses but in a time sensitive deadline driven project with the budget that would allow it, I would go with a 3rd party option than trying to use multiple expressions and multiple Ram intensive filters with CC particle world to achieve something Particular can do without as much hassle. Now, you could also purchase a template to save yourself the hassle if you don’t know how to pull that off but there is no guarantee you or your client would be willing to spend the money to get one. So overall, the whole point of the author showing you what you can do if you invest some money into some 3rd party plugins is that your creative bounds can be expanded to new horizons. At the end of the day, the choice is completely up to you.