That somewhat depends on if you are looking at Splunk Core, or specifically any of the premium apps like Splunk Enterprise Security (ES) or Splunk IT Service Intelegence (ITSI) or even Splunk UBA.
Assuming you are approaching this from a security standpoint the Splunk Security Essentials App
https://splunkbase.splunk.com/app/3435/ lists numerous ootb use cases, and the products they apply to.
you could start to see in the following urls:
then this video is very useful:
Thank you - those are useful, but is there a simple list of use cases listed anywhere that you know of?
They are updated very frequently, and are only (to my knowledge) completely detailed in the Security Essentials App (which is updated just as frequently)
Just an addendum - they are also detailed inside ES, but unless you have access to ES, that is of less help.