When I was young, most people I knew carried duffle bags. Though there were a few with backpacks, they weren't as common as they are today. When we went school shopping, we usually chose a duffle just because that was what almost everyone else was using. Today, most will choose to carry a backpack if given the choice, but there are still some great uses for the duffle style of bag. Some things just can not be carried in a backpack if you want them to remain in one piece.
A backpack is convenient for many reasons. The biggest is that they are so much easier to carry than the duffle bags. They are designed to be carried over both shoulders and on the back, but most sling one harness over one shoulder and carry them that way. This makes them easier to handle. However, there are some who know that the duffle bag is the smart choice in many occasions. A good example is a musician. Though there are special bags for some of the more sensitive equipment that they might own, many will carry that stuff in a duffle bag. If the same stuff were put in a backpack it would be broken rather quickly.
Though some like to use a briefcase to carry important papers, there are some who don't like the look. When they have a large amount of papers to carry around with them, they will find the duffle bags will help protect those papers because they will be lying flat. Think about teachers who have to carry a lot of papers back and forth to work each day, or the person who works from home for a few days a week. They have a lot to haul. Those same papers put into a backpack would be creased and wrinkled in no time. Backpacks are built so that the papers would have to stand on end, and that means damage.
Duffle bags may not be as easy to carry as a backpack, but they are useful for many other reasons as well. They are even a good idea for travel. You can't keep football equipment or most other types of sports equipment in a backpack either. They are often more versatile, and can cost less as well. Of course, when hauling something delicate around, the more expensive type of duffle bag would be the best investment.