It's not well thought-out.
The idea is to update host attributes by auto-registration actions - ideally with an option to set/update it only if unset.
One use-case might be to update the host attribute 'Visible name' with the short host name of a host.
In our case all hosts reside in a large hierarchy of domains and the FQDNs don't look very nice in the front-end.
Currently we have to edit every automatically registered host manually to achieve that.
This can be really painful in highly virtualized environments.
Adding or updating host macros might be another use-case for such kind of operation type.