[pjc50]

The advantage of the Arduino is that it's a standard that "just works" with absolutely minimal time investment of getting started. You can have an Arduino up and running with a flashing LED before you've even assembled the breadboard and installed the toolchain for another solution.

As a standard, users benefit from being able to share instructions that have a high chance of working on someone else's system.

Obviously these are most relevant for beginners or those with a short time to invest. But that's the market that Arduino practically created. It simplifies the decision making process: if you're a beginner, start with Arduino. Once you've used it for a bit, you'll be aware of the limitations and have an opinion on which alternative to switch to.