When I setup a Development Environment for a project sometimes it can take a few hours. VirtualBox is well suited for backing up this effort. Also if I am managing a team of Engineers I can specify the Development Environment they should be using by just giving each of them a copy of a VirtualBox instance.
I don't need to specify a large recipe of tasks they need to follow in order to get their laptop ready to participate in my project. Also I dont need to debug problems with their laptops due to differences. If one member wants to use a Mac and another wants to use Windows it should not be a problem if they can run VirtualBox.
Some apps need to run on a cluster of hosts. Sometimes I need to build a Development Cluster which is structured like the Production Cluster. VirtualBox is well suited for creating a cluster of hosts. If my laptop has lots of RAM and CPU, I can run the whole cluster inside my laptop.
When I have a class full of students, I want to spend time teaching. I don't want to spend time tinkering with student laptops.
So, I place the Training Environment in a VirtualBox. Then each student gets a copy. Hopefully each student laptop can run VirtualBox.
I need a Personal Computer to do tasks like send e-mail, listen to music, file taxes, surf the web, store documents, and watch Netflix. I use a VirtualBox instance as my Personal Computer.
Before VirtualBox, my Personal Computer was a laptop. Compared to VirtualBox, a laptop is a poor Personal Computer. The main problem with a laptop is that it is difficult to backup and restore. VirtualBox is easy to backup and restore.
Now that I use VirtualBox, I back it up once a day. I have restored VirtualBox instances many times; it is always easy. Restoring a laptop from backup is usually not easy. Now, when I look at a laptop, I see it as a thing which can run VirtualBox. I no longer use a laptop as my Personal Computer.