> Sometimes, he'd run into things which didn't make sense. Why is this a doubly-linked list here, when it seems a singly-linked list would do just fine? And in those moments, if you can't find a reason? You get to go down that path, make it the singly-linked version, and then find out later: oh, ohhh. Ohhhh, they did that for a reason.

Everything is done for a reason. Unfortunately, often the reason is the author didn't know what they we were doing.