One feature of my app is the PDF creation of invoices and estimates. I use a Java library to make these PDFs. Developing an iOS, web, and Android app that syncs in real-time is much easier (or even possible) when your PDFs are made on the backend.
Part of the primary power of cloud functions is you can extend firebase services instead of intermediating them. This lets you still take advantage of all the great features in the firebase SDKs.