Sub Flows

Workflow is better for running some actions in background process, such as sending data to external request for tracking, which may take a few seconds to complete, and the response is not required for the next message sent to bot user.

You will not be able to use any send message or ask questions to bot user inside workflow.

Function flow is better for reusable flow, which handles the common logic and returns back the results via output node. For example, you can use it to check the business hours and then route to different output based on the business hours.

also you can check if user has specific tag or not, and then you may follow up to different questions based on result. The key benefit is if you have may sub flows need to handle similar logic, then by using the function flow, you will be able to manage the logic from one single flow, it is easy to maintain and make changes.