From what I have seen, most developers aren't working on anything particularly interesting in their day jobs. At best they are developing a new CRUD application using the latest framework, but at worst they are maintaining legacy codebases, battling management incompetence etc. It can be very hard to sound passionate about these things in an interview. In my view this is where side projects come in - they're the chance to do something actually interesting, which can be a great talking point in an interview.