Yes, in AWS you will get one VPC by default. You can use this VPC for your work. But when there is a requirement to create your own network for your own purpose or your company, there you need to create VPC. Say in your company there are two teams. One team works in development area and one team works in production area. Now these two teams works in two different subnets, but they should work within the same network or VPC of your company. So that they can communicate and share their things. These kind of use cases you can think.
Hope the above given example clear your doubt.