Today, social coding movement more spreading around GitHub, people arguing while develop proccess.
that's absolutely good trend, because software development is huge complexity human work.

but i think a personal project has some effective things to grow as a developer.
following 2 are my thought.

full stack roles experience

All processes are become to your work.

whole product design, software feature, technology-stack, UI/UX design(if you create GUI app.), marketing, software architecture, code implementation etc...

yeah, your are completely full stack developer!

this fact useful to learn other role person's job overview, each process integration flow, approximately total implementation cost.

free experimental implementation

All work isn't depends on colleague opinion or your boss approval, you can feel free design, implementation.

you can failure Unlimited on the environment and test any methodology pros/cons.

what do you think?
thanks a hundred for your comments! 😁