The tireless Artur Malabarba has shown us how to make custom Org links for special purposes before (1, 2). Now he shows us how to implement Markdown links. The point here is that with Markdown, you define a link and associated ID once and can refer to it any number of times thereafter by mentioning its ID.
That’s nice if you refer to the link several times in your document. Instead of looking up the URL each time, you can give it an ID and then use that ID each time you want to refer to it. This could be especially useful if you later need to change the URL for some reason.
Malabarba’s implementation takes 3 simple functions, two of which are required by the Org Link mechanism. It’s pretty easy and could be really useful for certain types of workflows. Take a look at his post for the details.