Why a backpack, especially 40 liter backpack for one bag travel?

The number one thing is a backpack. A clamshell opening. 40L. Get something durable.
I have reasons for all of these things.

Advantages of a backpack

Easier to get around - If you need to run to catch a bus, you can do it. If you have a duffle bag, this prospect is much more difficult.
Hands free - You can grab a drink or food and not have to mess with your bag.
Active - If you want to bike, you can easily bike.

Look at everything at once - Compared to a typical top-opening backpack, with a clamshell opening, you can easily open it and look at everything at once. With a normal backpack, you need to dig to get things in the bottom.
Downside is that if you do open the bag, everything is visible. If you are on a train, it is easier to open a top opening backpacking and keep contents inside and discreet. With a clamshell, you can get around this limitation by having the things you likely need to access at the top of the bag (water bottle, jacket).

Despite using smaller bags on many trips, I have not gotten my ideal size lower than 40 liters. Here's why:

Extra space - I pack enough stuff that i easily fit it all in 20L bag. 40L is quite large, but the space is so helpful. You can use it to bring back souvenirs and gifts. Without the extra space, you would need to bring a second bag.
Fits in all overheads - This size fits in overhead bins. Especially if you get a soft-sided bag. It would also fit underneath the chair, potentially, but it will take up all your space.
Alternative: One alternative is a duffle bag with backpack straps. It is nice to have the opening be against your back because it is better for security. The duffle is very nice because it is even easier to get at stuff, plus it is discreet. Duffle bags hold surprisingly much for the perceived size.