We sometimes want to deploy data with the Lookup fields populated, without also updating the records that those Lookups point to.
The example use case here is that I 'cleaned up' some child records in the source, and made some changes to their parent accounts too. I only want to deploy the child records, but not the parents.
The child here is the Contact, Andy Young.
I created a configuration that specified that I wanted the lookup fields, but not the related records - to do this I selected the Contact, and the reference fields, on the first configuration page.
The contact was deployed, and it was reparented to the right account (in the target).
On the second page, I deselected the related objects (found on the right hand side of this page).
I was able to successfully deploy to the target - as you can see on the summary the only object I'm updating is Contact, which is the 'child'.
This is the Contact record in the target - the Lookup field to the parent Account is still populated, but the Account itself has not been altered.