[repsilat]

It helps for intrinsic motivation, but (in my experience) that normally isn't too lacking. Where it doesn't help is the motivating effects of feedback.

Having made a physics engine is cool, but having a working physics engine that makes the product fun is also rewarding, and a shortcut to that reward can pay greater dividends on the time to get to the finish line -- or the likelihood of getting there.

Often the shortest path is writing something yourself, of course, and not everything has to be done for instrumental reasons -- a hobby project can be its own reward.

I try to take a counterfactual approach -- say I'd done Box2D first. Would I think a custom physics rewrite would make sense? It'd still be really fun to do, but would take time and likely move the product backwards. (Though of course that's unfair -- you don't know the Box2D thing will be successful in the first place.)