Interruptions are frustrating, especially when you need to concentrate. However, you need to stop work at some point and talk with customers or other people you work with.
No matter what, you're going to get asked how something works, so you'll need to create or update documentation. Feature requests may come through in a specification, and bug reports in an issue tracking system or code repository. So many systems to distract you from your job, without the ability for a running conversation on all things in one place.
Cavy brings it all together in a single, agile, collaborative system. If you're a lone developer, it's a fast way to get things done. If you're a manager, you can bring your developers in on the conversation when required. If another customer asks the same thing, you can notify everyone automatically.