The permissions model of Linux means that, even if you are the only user of your computer, you're not the only user on the system. Many services will create their own user account to run under - for example, Apache will usually run under its own dedicated account.
What you will also notice is that your home folder is usually only accessible to your own account - i.e. permissions
drwx------. That means that only you can get at your home folder, even if subfolders within that folder have full access.
This combination provides a useful balance. The default permissions allow service accounts to read any files on the system that they might need to, but they can't change anything. Any files which are personal to you should be in your home folder, so a rogue service won't be able to access them.
You might find the Filesystem Hierarchy StandardFilesystem Hierarchy Standard a useful read. This outlines the generally expected permissions and functions for Unix/Linux systems, and most distributions will abide by this.