The problem with some of these files is that if you delete them they return a few days later. It’s not cool.
But how do they do that?
When a hacker breaches your site’s security they will often leave a hole for themselves to come back through. Even if you patch the security hole they will return because they’ve given themselves another way in. That’s why it is extremely important to upgrade your blog or CMS software whenever developers create a new one. Don’t pass those opportunities up. You’ll regret it.