From what I can tell, they’re creating the content to help users realize how to set up a separate front page and a separate blog page which isn’t always intuitive to newcomers to the platform. Fortunately they haven’t made too much useless content and new users can simply either edit the front page the theme creates to something that suits them or just delete that page and create something of their own.
The only other content I can see that one might want to modify are the two footer widgets that you can either edit or simply delete or replace.
In all I think they’ve taken this route simply to give new users an idea about how they could set up their sites and give them an idea about the way the theme uses the Gutenberg editor. (It’s not too different from the long standing “Hello World” introductory post or the “Hi, this is a comment.” first comment on a fresh install.) For people new to WordPress this is probably pretty helpful, though for older hands it may be annoying. Fortunately the content the customizer creates is pretty minimal and easy to get rid of.