What is Service Broker in SQL Server?

Service Broker is a messaging system built into the SQL server db engine. Here's some articles you might read to see how it works.

Centralized Asynchronous Auditing with Service Broker
Centralized Asynchronous Auditing across Instances and Servers with Service Broker
How to troubleshoot Service Broker problems

SQL Service Broker is an extension mechanism that allows you to queue events for asynchronous processing.

There is no intrinsic harm in enabling the broker. If it's not used, it will just be idle.

It works in both simple and distributed DBs. A simple use case would be a logging queue. We used it at a client to queue XML messages to be processed asynchronously. So we push an XML to an InitatorQueue, and then had a service pull them from the queue, extract some necessary attributes via XPath, and insert them into a persistence table in our database.

Here is a good reference from Microsoft.