Like Rodolfo said it checks if the script is installed, so as long it can read database and tables the website will load up, but if it detects that script is not connected to database or have any tables then it will mostlikely bring up the installer or a Something went wrong page. So install folder is not directly accessible by going to it by doing the example method domain.com/install or similar since doing so will just redirect user back to index page since script is already installed.