Monolithic code bases are everyone's worst nightmare. Microservices don't offer a cure-all for all large scale applications and come with their own set of challenges, but the potential benefits could be a huge boon for a fast growing start-up.
The first force that led to the surge in microservices was a reaction against traditional, monolithic architecture. While a monolithic app is One Big Program with many responsibilities, microservice-based apps are composed of several small programs, each with a single responsibility. This allows teams of engineers to work relatively independently on different services.