The different font families have specific benefits:
Serif fonts compared to sans-serif are easier to read on high-resolution media (such as printed books or 300 ppi displays), because the visual difference between the letters is higher.
Sans-serif fonts compared to serif ones look better, however, on low-resolution media and before technologies such as ClearType existed. With low resolution, small details are easily lost, which means that simpler fonts win.
Monospace fonts compared to non-monospace ones are great when one needs to either work with rectangular selections, or when mistakenly taking one character for another could have serious consequences. This is why monospace fonts are commonly used for development. Can you spot two errors in “const log = logger.create(global,appCorreIationId);”? Now what about monospaced font?
const log = logger.create(global,appCorreIationId);
The difference is subtle, but it is slightly easier to see that a dot was replaced by a comma, and a lowercase letter “L” became a capital “I” in “correlation.” For the same reasons, some fonts go further to differentiate characters which look very similar, such as
O: an oval with a dot (or a slanting line) is a digit zero, while a slightly differently looking oval with nothing inside would mean a capital letter “O.”
Last but not least, monospace fonts are used in some fields because of the predictability of how much text there is on a page.
Monospacing makes Courier a consistent representation of the “one page equals one minute” rule. If we were to use a proportional font, the mixture of spacing would make that rule less accurate.
Source: Why We Use Courier for Traditional Screenwriting Font
I'm not sure about the validity of the claim. Not only I'm not convinced about the predictability for a monospace font (one page of a movie script can contain lots of dialogues, which can be read in a matter of seconds; another page could contain a long description or a long monologue, meaning much more words per page), but I would like to see some statistical data to compare monospace fonts vs. proportional ones in terms of number of words per page. My impression is that proportional fonts would be more predictable.
Additionally to that, designers may use one font or another to lead to a feeling. Show a picture of a dark screen with light blue or white monospaced text going in all directions, and a reader immediately thinks about hackers. Here's an example from a well-known video game about hackers:
Monospaced green text on dark green background makes immediately think about mainframe terminals. Here's an illustration from a popular video game where the player finds himself in a parallel world with technologies stuck in the sixties:
And naturally, monospaced font, black on beige background, makes immediately think about typewriters. Here's an illustration from another video game where the story happens during the Second World War:
In your case, you identified that the users like the typewriter feeling. However, there are no technical benefits to use monospace fonts in your case:
- The readers don't need to do rectangular selections.
- They don't need to focus on typos either.
- And they don't need, I believe, to know how long would it take to read a block of text. If they do, you can add an algorithm which would show it at the top of the page—something a lot of blogs do nowadays. Example: “Aug 6, 2020 · 11 min read.”
Since monospace font is more difficult to read (take a long piece of text, say twenty pages, and print it using a monospace font; then print the next ten pages using a serif font, and see by yourself), I agree with your choice of not using monospace font for the long chunks of text, such as the description on your screenshot.
Now, when it comes to short pieces of text—headings, authors, labels, etc.—I personally find it strange to use a monospaced font here, but this may be explained by my background: I'm a software developer, and so most of the time I see short pieces of text using monospaced fonts, it corresponds to the pieces of source code, or system names of machines, or other technical stuff. Therefore, from my perspective, seeing “Ernest Hemingway” in monospaced font is very surprising; it's less surprising for the labels, and even less for the button, where I haven't noticed the font at first (I was more annoyed by ALL CAPITALS; don't yell at your users; you're not Steve Ballmer).
Persons with different backgrounds may actually find it cool to have small pieces of monospaced font. In order to find it, don't ask them straight what they like; instead, do two separate versions: one with monospaced fonts here and there; a second one with serif fonts everywhere. Then do A/B testing to determine which version is better.