A Temporary Chat is a clean, one-off conversation. It starts without your saved memories, doesn't add anything new to them, doesn't show up in your chat list, and isn't used for training.
Use it when you want an answer that isn't coloured by what ChatGPT knows about you, or for a one-off you don't want in your history. OpenAI may still keep a copy for a limited time for safety reasons, so it is not a place for things you shouldn't share at all.