your repo is a project, projects are served from a sub folder, with the repo name as the sub folder name. The baseurl is one way to specify that the urls are located in that sub folder.
Your main username can also be a website, it is at the root and would not need baseurl as it is not served from a sub folder. All GH projects that use GH pages need the baseurl to be set or the css and other things will not load.
This is fairly clear when you look at the url to that site:
https://brando90.github.io is the root of the site - whether or not you have anything there, but that is not where your repo is, so baseurl just adds that in by adding
/brandomiranda to the url.