Page "Web bug" Paragraph 24

Web bugs are used by email marketers, spammers, and phishers to verify that email addresses are valid, that the content of emails has made it past the spam filters, and that the email is actually viewed by users.

When the user reads the email, the email client requests the image, letting the sender know that the email address is valid and that email was viewed.

The email need not contain an advertisement or anything else related to the commercial activity of the sender.

This makes detection of such emails harder for mail filters and users.