Markdown, similarly to HTML is a markup language used to give structure to content. It is used for many things, from blogs to documentation pages to text messaging. Unlike WYSIWYG (what you see is what you get) editors that come and go, Markdown has been here for over 17 years and it doesn't seem to be going anywhere.
The main difference between WYSIWYG editors (e.g. Microsoft Word) and Markdown is that in the former you see what the text will look like to viewers right away, whereas in Markdown you add formatting elements to that text. You can later render it as HTML or use it as-is, because Markdown is pretty readable even it it isn't rendered.
All you need to do is add tags (#, >, **, ~, `, ...) to the text to format it. Here are some of the most common tags. You can see the real power of Markdown in action. Here's a reference if you want to dive deeper into the syntax.
# heading 1
## heading 2
> This is a blockquote
![image alt text](/img/path/file.png)
The main advantage of markdown in my opinion is its simplicity. You don't need any fancy custom text editor to create well structured content. It is so minimalistic that you can learn Markdown really in a matter of days. This gives it a massive advantage, because anything that makes creating content easy and boosts your productivity is a good thing to have under your belt. If you're accustomed to the syntax, you can write content really quickly.
Reliability is another big positive of Markdown. It has been here for over decade and a half and I'm pretty sure it won't be going anywhere in a near future.
Other big advantage is that you can convert Markdown to HTML using tools such as Marked for Node.js. This is great because you can format your HTML however you want with CSS.
Ease of sharing is also very important if you're working with other people. Remember, Markdown file is just plain text, so it can be opened by any text editor, such as VS Code or Notepad. You don't need fancy software to edit it, so you can share it with others and they can easily open it. There are also online Markdown previewers, so they can even see it as a nicely styled HTML.
There are also many tools that can help you write Markdown so you don't need to use your basic text editor. You can have custom themes, keyboard shortcuts to make headings, bold text and many other formatting options. Typora and Ulysses are both great options.
The simplicity of Markdown is a great thing, but because of the fact that there is very little to it, you don't have full control over your text. With traditional document editors, you could for example increase font size of only one word in a paragraph and make it other color. You can't do very complex things with Markdown. For the content where you need many custom styles, it won't be the best fit for you. In that case, it would probably better to just use WYSIWYG editor.
Markdown is being used heavily in the blogging space. Many blogging platforms and CMSs rely on Markdown for its simplicity and ease of use (Wordpress, Ghost, ...). Markdown is also an excellent fit for Documentation pages, because it's simple to write, maintain and collaborate with others using it.
If you're in a market for solid text writing tool that is easy to use and has a strong place even in this day and age, definitely give Markdown a try.