Hi there . You make a good point, why use another tool when Ghost does it all on it’s own? And yes, adding another moving part to the stack does add more complexity. The benefits lie in the aforementioned “stack”, and more specifically how developers can benefits from it.
Using something like Gatsby puts your web development stack in the JAMstack space. The benefits of JAMstack is that you can create a fast static site while still getting all the CMS form of editing and control you get when you source content from an API, such as the Ghost Content API.
It’s not to everyone’s taste. Think of it more as a way to get more control. With more control you can tweak dials, improve speed, improve presentation. The drawback is more moving parts and maintenance. I’m not going to try and sell the concept to you, because it’s just and option not an improvement. We want to give people the control they need over their site. Using the Content API with something like Gatsby is just one way
I hope this explains things a bit more. You might also benefit from checking out these resources too: