The reason is that you are more comfortable or efficient in the new language, but want to access legacy. Having to recreate everything for scratch is a massive

The reason is that you are more comfortable or efficient in the new language, but want to access legacy. Having to recreate everything for scratch is a massive effort, however gradually refactoring an application and having access to huge amounts of third-party libs is an enabler. (while all those libs will use non-idiomatic patterns, so it has to be contained)