Why should I save both markdown and the converted HTML in my database? First of all, forgive me if this is the wrong place to ask this or if this has been answered somewhere and I just can't seem to find it.

So you have a scenario where you have an article/news web app and your authors use markdown to generate content.

With regards to deciding database schema, everywhere you search, the recommended method is to save both the raw markdown text and the converted HTML in your database, but my question is WHY?

Internet: You never want to parse markdown every time you have to display the content.

Me: Yes, so parse the markdown and store the HTML into the database directly, so you don't have that performance hit when serving up the articles/content since you want speed here. Good, that's fine.

Internet: It makes it easier to edit the raw markdown later, so saving the markdown to the database is recommended.

Me: It sounds good in theory, but what happens when you have a very large database with tens of thousands of rows? You basically inflate the size of your database just to prevent parsing some HTML back to markdown when you edit the article/content? How often do you edit these articles? Not that much since most of the hits are just reads to display the content to your end users.

So my question is, where is the benefit of storing both the markdown AND the converted HTML in the database? The "later-edit" argument seems moot to me when just to save a minor conversion before editing, you're basically inflating your database size by needlessly storing the raw markdown in addition to the HTML. Your articles table can theoretically double in physical size just because of this. What's the point then?

It just sounds like pre-optimization that leads to a larger detriment later on when your infrastructure is forced to scale because of the duplicity of the markdown/HTML content. A conversion from HTML to markdown seems much more realistic to me, not like the authors/editors are going to notice the extra few milliseconds that the converter takes when they try to edit something.
Your argument is flawed.

You are concerning yourselves with doubling a 15kB per article  × 10k rows = 150MB table. (Excluding images)

And, you will be accessing individual rows via a PK.  You can have a trillion rows and the database won't care.  10k rows is tiny.

Additionally, the database is already going to be inflated due to Revisions and Text Indexing .  Adding the HTML version shouldn't hurt you.  Not even at 10M rows.

Oh, if the author is editing the Markdown version, you better record what she/he wrote.  (Ie always convert from the author's format to the printed format; never the other way around)