Many Mac users just get along fine with the macOS builtin Apps from Apple. I think Apple does a pretty good job with it’s default Apps, but some people want more features, better versions of these Apps. For example, I use Airmail to replace apple Mail and BusyCal to replace the Mac Calendar App. Enter this article which lists several Apps you can obtain from the Mac App Store to replace their Apple counterparts. Yes, these third party Apps cost a couple of bucks, but if you are an Intermediate to Power User they might be worth it to you.