Confused about the difference between using postman & a database

Hi all.

I’m a student working on a simple to-do project. I’m supposed to connect to a public API and create a postman collection. I also need to be able to add, remove, sort, & reorder the todo items. However, I’m quite confused about where Postman comes into play. It seems if I want to demonstrate this when I send it to my professor I’d have to have a database where she can run these operations on it to see that it works. Or am I missing something - should I not be using a db at all and just making fake requests for all the calls? How would that work for adding items?