Hosts define "endpoint" servers where MOVEit Automation can pick up files or drop files off. Sources and destinations directly depend on host definitions; they can override some information (such as the credentials presented) on a task-by-task basis, but the IP address, port number and other information must be defined in the underlying host definition.
Changes made to a host definition immediately affect all sources and destinations that depend on that definition. This is beneficial when configuring many tasks to go to the same host, especially if that host changes its IP address.