What are web design apps like Sketch and Adobe's Photoshop/Indesign Used for?

This is probably the dumbest question you read all day but I am generally perplexed by this. What are these apps used for?

For instance, I am trying to design a very basic e-commerce website for my database class. This website is basically a simulation of a bookstore website that also sells movies and music.

I heard good things about Sketch when it came to web design so I downloaded a free trial of the app and started using it not really knowing what it does. I designed what I thought was a pretty descent website. But, when it came to exporting it, I was just given options of exporting an image or pdf of the design.

So, are these types of applications just used to create images of what you'd like your website to look like? And if so, what's the point? Do designers who use these websites then take them to developers who type of the code for the site in HTML/CSS/Javascript based on the design they are given?