When an environment is read-only, any versions deployed to this environment will permanently become read-only. This setting protects sensitive environments from accidental changes.
For example, Production is a read-only environment by default. This setting prevents another member of your account from accidentally editing the version associated with your live traffic.
For similar reasons, some organizations may make Staging a read-only environment. Otherwise, another member of your account could make changes to a version in Staging after your organization has performed the validation tests prior to promoting to Production. Without having a read-only Staging environment, this change could be released into Production without testing and might cause an issue with live traffic.