I don't use this app, but I do use 1Password, and have thought about why I use something in addition to Keychain. What it comes down to for me is that I can keep other stuff securely in 1Password beyond just notes, and it's indexed and smart and everything. I actually find that I use it more for those other things these days. Additionally, it has had sync before Keychain had good sync, and I control it (and more importantly, can back it up on my own).