Is your email client really set up to phone home to the sender to retrieve additional images when it opens an email? Doesn't that strike you as an information leak/privacy issue?
Mine (Gmail) is not, but many (Outlook, OSX Mail) allow this by default. The URL of the tracking image contains a number or hash that is used to uniquely identify the person opening up the email.
Outlook hasn't done this by default for at least two if not three versions now. However if you read a heavily stylized with images email you'll almost certainly tell it to download images (just as one does in gmail), giving them that feedback that they want.
Good to know, it's been several years since I used Outlook.
People tend to enable images on newsletters because many of them are unreadable without them.
https://emailprivacytester.com/ seems like a way to check how your usual clients fare with these various tracking methods. Or a cunning way of snarfing emails, I dunno.