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).
I always wonder about that when people talk about these things. I think the reason is that Keychain Access isn't well advertised and a bit obscure, so most people are simply not aware that there is already a thing that does that on every Mac, with (optional) cloud sync and a standard API used by almost everything on the system (including ssh and Safari -- and Firefox with an extension).
For one, iOS keychain won't let me see items itself, or their full descriptions. It only allows to paste Safari website passwords. At least I haven't found how to do more than that.
From screenshots, it looks like this app would let you browse your whole collection in full. So there's at least one feature.
iCloud Keychain is handy for simple use cases. But it quickly breaks down if you want to have a good password habits.
For example, I use my Apple ID to login to a bunch of different Apple sites. With the keychain that would create separate entries for each site although they are the same. Change your password and you'll end up with items with outdated passwords (which you'll only find out when you try filling them).
The keychain is also cumbersome to create items manually (imagine you need to save an SFTP or VNC login?). Furthermore how would you have access to these items on your iOS device?
You also can store more than just passwords with Secrets.
Recent versions of Keychain on OS X/macOS (and iOS I think) ask if you want to change other entries with the same username from the same domain (i.e. appleid.apple.com and developer.apple.com) when it detects a change.