It is easier to scale applications that are limited in scope. One reason for that is that there are fewer potential bottlenecks.
Scaling smaller pieces of a workflow is also more cost-effective. Imagine a monolithic application responsible for managing a trade fair. Once the system starts showing performance issues, the only way to scale is to bring in a bigger machine for the monolith to run on. This is called vertical scaling.
With microservices, the first advantage is that you can scale horizontally, that is, bring in more machines instead of a bigger machine (which is usually cheaper). The second advantage comes from the fact that you only need to scale those parts of the application that are having performance issues. This also contributes to money saved on infrastructure.