In my day... mail was delivered seven days a week. In large city, there were often three or more deliveries a day. Today, once a day, six days a week, and probably will be cut even more.
The Post Office department was one of the first things set up in the early days of our country. Benjamin Franklin was the first Postmaster General. Why was this a priority? Because it was the most important means of communication at the time, even though it could take weeks for mail to travel to the farthest of the colonies.
It is still important these days, but slowly losing its importance to other media. Letters and parcel post still account for most of the revenue, and direct marketing (junk mail) comes in third. Technology has always led the speed contest for communication. As the railroad network developed, mail speed increased. There were special railway cars that allowed sorting and processing to make things more efficient. But getting mail out to the west was a challenge, and the Pony Express was created to allow for a much faster and more expensive) way to deliver. But it only lasted less than two years, as the telegraph made communications nearly instant, and railroads expanded. FAX machines started displacing mail in the 1970s, but were soon eclipsed by electronic mail and other forms of internet delivery.
We still use physical mail for documents and packages that need to be delivered in a physical form. The cost of delivery has greatly increased over the last few decades, and the speed of delivery is slowing down. But for most of my life, mail was very important not just for personal communications, but for also commerce. Today, we order items online with a few clicks. Let's go back to 1964, when I was working on an electronics project and needed a rectifier. I sent a letter to Allied Radio, requesting a catalog. A few days later, I found what I needed and put my order and payment in an envelope. In a few days, they received it and sent my order. By the time I received it, two weeks had passed since I requested a catalog. Now, I would go to the web site of DigiKey, pick out what I wanted, click and pay, and receive it a few days later. It might come via the postal service, but just as likely as more efficient carriers like UPS or FedEx. Amazon, who ships lots of orders each day, created their own delivery fleet and offer next day delivery for much of their products.