When importing an XML file into a site that already has existing products, the importer needs to know how to handle data with the same identifier, usually the name. Each type of data object that the importer recognizes can be handled differently. These settings should be revisited whenever a new batch of products are imported to make sure the module is going to behave as expected.
The last setting is used by the Repeater module, which imports data programmatically. Setting the imported nodes' author to an existing user is more of a stylistic choice than a technical requirement.