Like with the gardening and cooking example above!
You might be able to build a perfectly wonderful garden, but that doesn't mean you can use the vegetables you carefully cultivated in any meaningful way in your kitchen to cook a healthy and delicious meal if you can't cook at all (but you can still share the vegetables with your neighbors who might share their cooking with you)! :)
Learned a lot that gets you a paid position at an engine company that makes a lot more than 99.99% of indie games (probably makes more than a gameplay programmer too). "When in a gold rush, sell shovels".
When you make yet another toy game engine, you've hopefully learned a lot, but what you have is a lot of dead code.