Loading a dataset allows you to pull the query being used from an Analytics dataset.  This can be helpful as you can use a UI-driven tool (the Analytics dataset) to pull all of the fields/joins and then pull that query into the NimbusQL application for further use.
Additionally, the Analytics functionality in NetSuite does not allow for scheduling emails.  If you would like to use an analytics dataset to send emails, you can use the NimbusQL application.