As per my understanding it is user who has access to publish the specific page/resource.
Documentation goes like this:
Depending on the environment, the agent will use this user account to:
collect and package the content from the author environment
create and write the content on the publish environment
Leave this field empty to use the system user account (the account defined in sling as the administrator user; by default this is admin).
means this replication agent comes into action only when replicating the content from packagemenager(by clicking replicate for specific package) ? or activating the page/resource from siteadmin?
The Agent User ID property is used to manage what part of content tree will be replicated using given replication queue. This has nothing to do with actual package creation - it applies to all replication process.
For the complicated infrastructure it may happen that the multi-tenant architecture involves some sharding approach. Imagine a geo-spread architecture with no CDN involved where the brand site should be quickly accessible from the given localisation. Due to technical limitations, pushing whole content (all sites) around the world might not be acceptable.
When DAM storage is shared across multiple AEM implementations it is often desired to dettach that from the regular authoring by creating a separated DAM-only instance. On such platform the replication agents should be configured to have the read access to /content/dam only in order not to mess up with other content trees.
In this case, the user agent ID can be configured to use a dedicated user permission scheme. All the changes the preconfigured user sees will be replicated to the corresponding endpoint. There are technical alternatives like implementing a transport handler (see https://github.com/Cognifide/CQ-Transport-Handler/blob/master/README.md)