Well for example imagine your stack is in Go and enjoy the speed and maintainability and expertise, but you want to use spAcy and all the great data science and ML libs in python. Or imagine you are all in on Java and want to use PyTorch. Or you are struggling finding an api-gateway like solution and want to utilize Zuul. Or you want to use Edge Lambdas on AWS but have to write them in node.js. etc.