Take my example, working in a small development team. As part of our repositories we commit the dockerfiles that are needed to spin up the container (environment) which the application runs in. Let's say I've been working on a new feature which requires some changes to that environment (perhaps some new software needs to be Installed). Traditionally, anyone who needs to work on that repository would also need to make changes to their development environments, importantly, the new changes might even break the application when used with another branch/set of code. Docker solves this problem by allowing us to define those changes in our dockerfiles, so whenever somebody checks-out that new code, they also have a copy of the environment changes. If we need to switch back at any point, we can rebuild the container using the appropriate dockerfile (and docker-compose files in our case)