What are some good use-cases?
Taking screenshots and marking up existing websites is good for breaking down the basic design and structure of a site.
Say if you’re building a website for a bike shop, take a look at various bike shop websites. See what speaks to you, then take a screenshot and mark it up. Now you’ll have a general sense of what to go for and can start building. Maybe even take that design and make it better. Maybe you only like the navigation bar from one website but you like the main section of another. You can mix and match different design sections from different sites and sort of Frankenstein your own design.