To use the email header, entire clues will be in the same area such as in a subject, name of the sender. Some senders will hide their email header. It won’t be same for the emails whoever the sender may be.
As said, for all the emails the unique header will be configured. In that case, you definitely would want to identify the sender and so to identify the IP address of the sender is needed.
Sometimes, you may get emails from PayPal to update your personal information. This way, we can find out the client of the email, the domain from which originate, address which you want to reply. The usual method has a choice to find the sender using IP address but there also another method to find the sender by email trace which is using email header.