Things like AWS Cloudformation require YAML input, so there's no real choice on what you emit.

But writing the YAML is fiddly and annoying, so that's a good example of something where it is better to generate it via troposphere (a python module) or some similar system.

To be less specific I guess the answer is that sometimes you don't control both ends - the part that emits and the part that consumes, and having faught ansible, and similar tools, if I can avoid it I'd never want to write YAML by hand for non-trivial purposes if I could script it instead.