I agree, but there are good reasons why it is often good to combine both types.

Considering this is only a side project, you have a high likelihood of abandoning it out of boredom, lack of traction or simply burn out. However, if your side project is "interesting" to you because you are experimenting with various tech, you come back to it because it is fun and it will keep you motivated.

It is totally cool to build something to learn and throwaway, but why not build something to learn that might also generate some income in the future?