Hi Steve,
Both the activities are used to query database based on a filtering criterion and feed the output to the rest of workflow.
Query activity will simply pass on the entire output but Incremental activity will match the query data against the data it has sent out in past and only the new ones are allowed to move forward.
Incremental query comes handy when you have to send out a welcome email to all new joiners. You can simply put the filtering criteria to say recipient created on or after 18th May 2017 and then schedule it to run every day.
On 20th, it will fetch records created since 18th May but then compare it against the output created on 19th May and pass only the ones that have not been contacted till now.
Regards,
Vipul