What's the difference to say between "by himself" and "for himself?"

Native speaker’s answer
Rebecca
If you do something "by yourself/oneself," it means to do it alone! Whereas "for oneself" means to the purpose of oneself. So here, "for oneself" would mean that the father lives for the purpose of himself and no one else. To live "by oneself" means to live alone. Ex: I drove by myself this morning since I got my license yesterday. Ex: She went on the trip for herself. She needed to relax. => for the purpose of oneself Ex: She went on the trip by herself. => she travelled alone