Protect your emailer reputation score
All companies that send email strategies are appointed a sender reputation score. It's a moment like a credit score. If you send a lot of bounced or unwanted emails, this will leave a negative mark on your sender history. Also, just like credit history, once your reputation drops, it can be tricky to build it up again.
Save money and feel better campaign results
Verifying your list means that you'll no longer send to dead email addresses. With these removed, your open and click-through rates are going to give you more information into your users' actual interests. Plus, having fewer customers on your list will help you save money and improve ROI.