If you're already familiar with setting things up Salt/Ansible/whatever and Docker compose, you might not need something like this -- especially if you're already using a dashboard like Dashy or whatever.
The biggest thing is that these types of tools make it a lot easier to set things up -- there are inherent security risks too if you don't know what you are doing, though I argue this is a great way to learn (and it isn't a guarantee that simply knowing how to use Salt or Ansible or another infrastructure as code tool will mean any of the stuff you deploy is any more secure) and a good entryway for people who don't want to do the Synology thing.
I like these sorts of projects because even though I can do most of the stuff manually (I'd obv. automate using Ansible or something), I often don't want to if I just want to play with stuff on a box and the app store nature of these things is often preferable to finding the right docker image (or modifying it) for something. I'm lazy and I like turnkey solutions.