I feel like one of the major benefits is being able to use the right language for the job. I can't be the only one who cringes at the thought of writing a large scale webapp in js, but writing my rendering logic in python on the backend and again in js for the frontend breaks dry. With a SOA you can split the backend into an API and have a presentation/routing layer in js that sits in front of it. This can use something like react to emit plain HTML from the server, and then that same react code can kind of re-bind to the generated dom on the client (using the exact same backend api!) giving you a single-page-app experience with graceful degradation and fast server side rendering for nearly no extra development cost. A guy can dream...