The question is: do we need flexible fonts? Other than fine-tuning the monospace font to fit your exact preferences given the hours you spend coding, everything else seems to be a matter of just picking an appropriate type that fits your project
As a designer, absolutely. The exact weight of a font can make a ton of difference in a finished product, and it's frequent to be frustrated that the font isn't available in the weight that would look right for the needs -- you need something in between semibold and bold, or something in between regular and condensed, or something oblique but not that oblique.
It's the difference between a design being good, and a design being beautifully "just right" with everything perfectly in proportion.
So for that, variable fonts are a godsend.
That being said, I've never seen a slider between proportional and monospace before. That's just weird. :)
https://www.myfonts.com/fonts/linotype/neue-helvetica/
Also, Helvetica is popular in large part because of the very precise personality it has with is quite amenable to the image corporations often like to portray. It become widely available on computers, starting with Macs, because it was already such a useful font. Major corporations aren't choosing Helvetica simply because it's a default font on computers. And, of course, it's not like a majority of companies use Helvetica for their branding or anything -- it's a popular font, but where popular means maybe 3% or 5%, not 50%.
Helvetica was put on Macs because it was so popular as a useful font, rather than it becoming popular because it was on Macs.
If I want to show something that looks like code - like an HTML tag or something - at a heading size on a page or a poster, a proper monospaced coding font is going to look bad. Playing with the MONO slider on https://www.recursive.design/ for a string like '<blink />' does a good job of producing something that looks like code but isn't swamped with too much whitespace...
Take two was Adobe's Multiple Master extension to Type 1. Technically this is still used by Adobe Acrobat to handle fallbacks for missing fonts, but as an end-user technology it was a failure, largely because of difficulties in being able to use the interpolated fonts—few applications allowed direct selection of interpolated designs and users had to generate them in ATM instead.
So now we have the third attempt, with better application support, which has some hope of success. For text type users, the appeal is the ability to finally regain optical scaling for type designs. Traditionally, as a type design was created at a smaller size, it would become relatively darker with less contrast. Perhaps the most dramatic example of this in everyday use is to compare the spindly appearance of "fake" small caps with the more robust appearance of properly designed small caps. So this is one case where, yes, we really do want this ability.
The HN audience loves to discuss the bloat in web design -- If you can reduce 500kb of font-<heavy, bold, italic, light> files into a single file that interpolates between font vectors dynamically, that goes a long way to avoiding excessive resources. And you also get the benefit of being able to tweak the parameters of each font style without changing out static files.
For example, with Recursive the entire static font family come in at 12.8mb, while the dynamic font family in a single file is 1.45mb. Obviously you wouldn't load up every variant and Recursive is a very large font, but I could certainly imagine replacing ~8-10 font files with a single variable font.
Generally with web design, you want to stick to a small number of font weights anyway so if the file size isn't smaller, what's the core motivation?
The key thing also is the number of web requests. This is not a trivial detail. One variable font can be less web requests.
Is the number of web requests a factor if you're using HTTP/2 though?