Splunk Add On ensures data ingested to Splunk is normalized and also a lot of other other performance, re-usability and correlation related features like Data Models, Saved Searches, Pre-built Panels etc to be used across specific and/or several Apps (when built upon CIM).
Splunk Apps are more User/Developer facing interface built upon such Add On which caters to specific Business needs/use case. Add On is not always necessary for Apps to function but CIM compliant Add On ensure faster/better App development and re-usability. Read about App and Add-On https://docs.splunk.com/Documentation/Splunk/latest/Admin/Whatsanapp.
Since these are Splunk Built and Certified. You can get assistance for issues with these from Splunk using your Splunk Entitlement. Obviously, the Splunk Answers Community can also assist with any hiccups you face while using these or any other issues with Splunk. Hope this helps!
Thanks, we can always have multiple answers to the same topic but I have generally noticed that once there is 1 valid answer it's much less likely that someone will feel the need to post another answer...(unless they want to clarify the answer or similar).