How do you get lots of engineers to work on a single monolithic application, while releasing the app into production on a very frequent basis (e.g., daily)?
The traditional answer has been to add complexity into your workflow (e.g., feature freezes, staging environments, code reviews, more testing). These are all reasonable things and good ideas. But at some point your velocity starts to slow down dramatically because of all this complexity.
Worst of all, this complexity isn't necessary for every feature. You might be prototyping some stuff, for example. But a monolithic architecture forces everyone onto the same workflow.
Microservices enables different workflows for different features (services). The value of this, even when you're at a small scale, can be pretty significant.