They are in addition to a simple Docker or Docker compose setup, not an alternative or replacement. If you want to, you can still use the simple Docker setup in a project for yourself even if it has a devcontainer configuration.

I.e., you can point a .devcontainer at a Dockerfile that you've already got working for your environment and specify standard configurations for environment variables, vscode extensions, and other bootstrap configurations so that the startup friction for a new developer working on the code is reduced to the minimum and the manual steps they need to compete to get started are documented and standardised in the devcontainer format, but you don't lose anything by doing so.