It's called Informed Delivery… and it allows you to see photos of the mail that's on its way to you. The United States Postal Service is calling it a service enhancement test.
You will receive images of the mail that will be placed in your mailbox each day. Black and white images of your actual letter-sized mail pieces, processed by USPS sorting equipment, will be provided to you each morning. Flat-sized pieces, such as catalogues or magazines, may be added in the future. Participation is limited to certain ZIP Codes ™ at this time.
Get up to 10 mail piece images in your morning email, which can be viewed on any computer or a smart phone.
For some, it's a reassurance that their mail is not lost and is safely in the hands of the USPS. For others, it's unnecessary.