I use Flask for times when I need to create a REST API that interfaces with other Python code, or there is no specific reason why I'd want to use Node.JS. I prefer Flask because of its small learning curve, allowing me to get started coding as quickly as possible
Paw allows me to interface with an API prior to starting UI work that requires the API. This helps me understand what data is required to be sent to the API, and what to expect back.
In cases where I develop the API, Paw helps me to test as I'm developing, ensuring changes I make aren't breaking other parts of the API.