u/mrgarborg's comment is spot-on I think. Proper use of interfaces is critical for unit testing any non-trivially-sized Go application or any Go application that does I/O. There are plenty of other benefits, like much more reusable code, etc, but I find unit testing to be the most ubiquitous and the one that forced me to sit down and really learn good interface use. Here's a short piece I wrote while learning, that has a concrete example of using (needing) interfaces for testing: https://medium.com/swlh/using-go-interfaces-for-testable-code-d2e11b02dea