HTML stands for HyperText Markup Language. It’s what the internet is built on. But it’s complicated and unreadable.
So Markdown was created to make things simpler.
For example, all I have to do for a Markdown file reader to know something should be italicized, is surround it with asterisks (i.e. *italics*).
To implement that same style as done above in HTML, I would need to have the following:
<p>…something should be <i>italicized</i> is surround it…</p>
Much more noise.
Markdown is simple and effective for writing, and it is transferable and readable in may applications.