There's nothing wrong with using an engine at all. For the same reason I use Numpy and don't bother making my own array manipulation classes. Most of the time

There's nothing wrong with using an engine at all. For the same reason I use Numpy and don't bother making my own array manipulation classes. Most of the time you don't need to reinvent the wheel because someone has probably spent a PhD implementing it better than you ever will. I think this is a perfectly valid excuse and I use it every day. If I didn't, I'd get no work done. With the libraries available few people should ever need to implement an FFT, for example. However, it's good practice to try coding some algorithms yourself to see how they work. In terms of games, I assume the intent wasn't to belittle, but to try and show people what happens under the hood. It helps you make better decisions about which engine to use and what features to use within the engine. If you're given pathfinding options within your engine, to take a silly example, do you know enough about the algorithm to know which one would be best for your game instead of blindly choosing A*? Understanding how the underlying code (probably) works allows you to make better decisions about which engines to use, which features you want and which you could brew yourself if you needed to.