We use it in development to match production environments
We ditched Heroku + S3 for dev/test/staging branches (if your app can be expressed in a docker-compose.yml file you can get a server up with the command `b3cmd server-scaffold`)
You can use it to run desktop apps if you're as anal as me about having a clean host machine. I also use it to run all of my small services (my website, IRC bouncer, etc).
I use it for dev to more closely mock prod, as well as ensure repeatable builds. It allows me to run all the services we use locally without a separate vm for each.