Rich text, or formatted text, is probably just one level above plain text. It does not contain full fledged images and other media like a word processor, but it can make your text formatting more evident. In other words, bold text looks bold, italic text are italicised rather than being plain or surrounded by **asterisks** or _underscores_ like in markdown. This can at first appear convenient, but like many conveniences it comes at a cost that might not always be worth it.

Formatted text is especially not plain text. Here’s a quick exercise you can do on your computer to understand why. First open a plain text editor (like BBEdit on macOS) and write down this sentence:

This is an **example** of how *files* are stored.

Save it as a plain text file, say with the markdown (.md) or text (.txt) extension. Then open a rich text editor (like TextEdit on macOS) and write the same thing, but this time make sure to format the word ‘example’ in bold and the words ‘files’ in italics as shown below. Save it again, but this time with the rich text (.rtf) file extension:

This is an example of how files are stored.

Finally, open the rich text file (.rtf) in your plain text editor, and notice how your computer actually saves your file. That plain looking sentence with bold and italics are saved like this:

{\rtf1\ansi\ansicpg1252\cocoartf2867
\cocoatextscaling0\cocoaplatform0{\fonttbl\f0\fswiss\fcharset0 Helvetica;\f1\fswiss\fcharset0 Helvetica-Bold;\f2\fswiss\fcharset0 Helvetica-Oblique;
}
{\colortbl;\red255\green255\blue255;}
{\*\expandedcolortbl;;}
\paperw11900\paperh16840\margl1440\margr1440\vieww11520\viewh8400\viewkind0
\pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\pardirnatural\partightenfactor0

\f0\fs24 \cf0 This is an 
\f1\b example
\f0\b0  of how 
\f2\i files
\f0\i0  are stored.}

By comparison your plain text file was stored exactly as it looks on your screen. With no intrusions at any stage, your plain text file is more reliable, meaning it is less prone to getting corrupted or requiring special software to open. While rich text is hardly ‘special’ in this day and age, it is still a layer on top of your text files and the more the layers you have the greater the chance your file gets corrupted.

Another consideration: rich text files are heavier by quite a lot compared to plain text files. If you check your file system you will find that the files in our exercise above were ≈50 bytes in the case of the plain text file and ≈500 bytes for the rich text file i.e. ten times heavier.

It is often worthwhile jumping straight from reliable, sturdy plain text files to word processors or finalised PDF files rather than bothering with the minor conveniences afforded by the rich text format.