Good question, Although the numbers are almost arbitrary there are a couple of reasons:
-- when looking at logs, database dumps or large forigen text files in general, it is often convenient for me to set the font to a smaller size in order to get better context from surrounding text and for this purpose, I need the font to be able to facilitate my workflow regardless of size and resolution of my current display.
-- Although I no longer use mini map like features of text editors (in part due to the novelty wearing off and in part due to my move away from VS Code as my primary IDE/Editor to Neovim), such fonts make the mini map almost readable on large high resolution displays (which again, provides better context for the code I am editing).
-- It is a handy measure of readability. If the font is readable at that size then it has good geometry and can be used on most low resolution displays (it is a range I came up with after experimenting with a 768p 15.6" laptop display) as previously mentioned sometimes it is handy to be able to lower the size in order to get a better view of the context.