A plain email with an attachment tells you nothing once it's sent: opened or not, you'll never know for sure. So every follow-up goes out blind, with the risk of sounding pushy about an email the client hasn't even seen.
Email read receipts aren't enough
Email read receipts are unreliable: plenty of mail clients block or ignore them by default. A real open confirmation has to come from the place where the client actually views the delivery, not from their mail client.
A delivery page changes everything
When the delivery lives on a dedicated page instead of an attachment, every visit is recorded server-side, without depending on how the client's mail app behaves. That's reliable data, not a guess.
xFer shows the status of every delivery (sent, opened, downloaded, approved) right in your dashboard. You know where each client stands at a glance, without ever having to ask.
Match your follow-up to what you know
If they never opened it: the problem is the email (subject line, spam, wrong address), so resend rather than commenting on the work. If they opened it but didn't download: the content or the clarity of the next step might be the issue. Two different problems, two different follow-ups.
In short
An email read receipt isn't enough; a delivery page that tracks opens does it reliably. Following up with real information, instead of blind, changes the very nature of the follow-up.