By "theoretical computer science topics", I am referring to things such as regular vs non-regular languages, the pumping lemma, and grammars.
I'm familiar with the real world applications of finite automata and regular expressions, but topics such as these other ones are giving me more problems as I'm not seeing any real world applications.
These things are useful if you want to know whether trying to do something is futile with regular expressions. For example, knowing that XML is non-regular is useful if the idea to parse XML with regex ever enters your mind. And if you don't know off the top of your head that XML is non-regular, the pumping lemma will let you prove it trivially.
And then there's compilers, where if you don't know this stuff, you're doomed. It just depends on the application.