How many times have you sent an email with a Read Receipt to ensure you know when the email was read and received no reply and no read receipt?
The truth is that it happens all the time and is probably more common than you think.
Many email clients default to present a pop-up to the recipient, asking them whether they want to send you a read receipt or not. More often than not people only send them when they don't want to offend the sender, which isn't the case for many of the emails they get daily. This is biggest problem with read receipts. There are also some email clients that don't support the sending of read receipts and those are the reasons why other forms of open-tracking are helpful when trying to get the complete picture.
Email tracking services are another way to track your email and be notified that your email has been opened. They use a completely different system to determine whether the message has been opened and it doesn't rely on an action on the users part in order to receive the information. That being said, there are also some email clients that block that method by default, so using one in conjunction with the other can be the best way to make sure for those really important emails.
One advantage that using a tracking service brings is the ability to track your hyper-links in your email as well. The reason this is important is because while both methods of ensuring that the email has been opened can be blocked or circumvented, there isn't a way to block links from being tracked. If you set things up right, you can always know whether or not the contact actually clicked through to the information you sent and for emails where you have attached more information it is a great way to know for sure.