Mail helps you track when your emails are opened. If you send an email about a proposal to a prospective client or an important email to your colleague, you will get a real-time notification when your email is opened. This would help you take action accordingly without speculating or asking if they have read the email or not.
When you compose an email, the 'eye' icon can be toggled to disable/enable open tracking. It is enabled by default for all emails you send.