Why is APM Server a separate component?

The APM Server is a separate component for the following reasons:

It helps to keep the agents as light as possible.
Since the APM Server is a stateless separate component, it can be scaled independently.
Data is collected in browsers for Real User Monitoring. APM Server prevents these browsers from interacting directly with Elasticsearch (which poses a security risk).
APM Server controls the amount of data flowing into Elasticsearch.
In cases where Elasticsearch becomes unresponsive, APM Server can buffer data temporarily without adding overhead to the agents.
Acts as a middleware for source mapping for JavaScript in the browser.
Provides a JSON API for agents to use and thereby improves compatibility across different versions of agents and the Elastic Stack.