Reaper has over 5000 separate actions and over 8000 if you have installed SWS extensions. Yet it can easily happen that user might want to do something specific which is not possible with any single one of them, but can be done with a custom action or a script.
So the ability to make custom actions is a really helpful skill and is not hard to do once you understand the basics of it. Reaper User Guide 6.21c, 15.16 Creating Custom Action Macros, page 284. And Kenny Gioias video,
Custom Actions in REAPER.