There are probably two use cases.

Configuration files for programs. These tend to be short.

DSLs which are large manifests for things like cloud infrastructure. These tend to be long, they grow over time.

My pet hypothesis is these DSLs exist mostly for neutrality - the vendor can't assume you have Python or something present. But as a user, you can assume just that and gain a lot by authoring in a proper language and generating YAML.

See https://github.com/cloudtools/troposphere for a great example for AWS CloudFormation.