Maybe they don't overlap 100% but, you don't have to pay or login to view questions and answers, and if you post a question in the wrong SE site it can simply be transferred to the correct SE site where the people familiar with that topic can answer appropriately.
Once upon a time I found myself contributing to StackOverflow and Programmers sites just to spite Experts Exchange, which to my dismay kept coming up in my search results instead of questions on a Stack Exchange site. It's consistently been a source of answers to questions that I thought were going to be hard to search for.
Maybe they don't overlap 100% but, you don't have to pay or login to view questions and answers, and if you post a question in the wrong SE site it can simply be transferred to the correct SE site where the people familiar with that topic can answer appropriately.
Once upon a time I found myself contributing to StackOverflow and Programmers sites just to spite Experts Exchange, which to my dismay kept coming up in my search results instead of questions on a Stack Exchange site. It's consistently been a source of answers to questions that I thought were going to be hard to search for.