으니까

Because, since

Gives the reason, and in these lines it usually comes after the point rather than before it, tagged on as an explanation: because you didn't do anything, because why not, because we see him on TV all the time, still, I did study Japanese. That trailing position makes it sound like you are justifying yourself. Unlike 아서/어서, it can be followed by a command or a suggestion, which is the difference textbooks test you on.