To my understanding the main point of an onion address is that they are free to be created yourself and publish within the Tor network. They require no central trust or regulating authority to setup and publish, therefore, in their very nature, help promote a free and open internet.
With "clearnet" sites, or, more specifically, sites registered under one of the ICANN top level domain names, we all rely on central regulating authorities. This involves mostly likely a fee that can vary widely depending on your domain name of choice. Also you are having to trust that the third party does not give your domain away to someone else for whatever reason they may choose.
As a web developer myself I find it pretty clever. I'm still new to this space but if I have been reading correctly I can publish my own website on the Tor network from my home PC, that I can then access from anywhere on the internet (once I connect to the Tor network from out in the world). All without having to register my domain to some third party, and pay yearly fees, as well as avoid paying for an SSL certificate (because I can generate the certificate for my onion site myself). As long as I don't overly care about the address of my personal site (which I intend only for personal use) this is a great, low to no cost approach to having a website ran from my home machine that I can then access elsewhere.
Use cases of an onion site are whatever you want it to be. Its advantage is that it is free to create. Its restriction is that it is always 16 characters, lowercase with numbers 2 through 7. And you don't have (too much) control over the name of it.