Using environment variables in your application (Deployment) via ConfigMap poses a challenge - how will your app uptake the new values in case the ConfigMap gets updated? You can obviously recreate the deployment - but this is undesirable in most cases.
However, if you load ConfigMap contents as a volume, you get the auto update capability for free i.e. Kubernetes ensures that the volume contents (files containing the config value) are refresh if the ConfigMap gets updated.