I am a font designer and I constantly use the Application font folder in Adobe Illustrator. The folder I'm referring to is cited here in the section "About Fonts":
My workflow involves editing glyphs in my font editor, then exporting the font files to this Adobe font folder. This is important because I can test small changes in my fonts very quickly without needing to install and uninstall fonts to and from the system (which often requires a restart.) We're talking about hours and hours of time saved thanks to this folder. It's vital to my work.
There are other benefits to this folder. Say you've purchased a new font and you're not sure if you're going to use it very much, or just want quickly to test it on a single project. You can drop the files in this folder and test it out in Illustrator without installing the font in your system.
Is there a chance of seeing a similar feature in Affinity Designer? This feature is the only reason I still use Adobe Illustrator. Without it, though, I cannot fully make the switch to Designer. None of the competition offers this feature either.
Thanks for your consideration.