For me, it's the fact that I can create a custom build of Iosevka with preferred weights that look good in the different font rendering systems: Native macOS, Electron (VS Code), Java (IntelliJ).
All need different weights to end up looking about the same on the same system.
And then being able to turn on/off ligatures, tweak some glyphs.
Link for reference: https://github.com/be5invis/Iosevka
Re: "SS04" d1egoaz is referencing Stylistic Set 04, illustrated here: https://raw.githubusercontent.com/be5invis/Iosevka/master/im...
In addition to the original version of the font above, there is a patched version with powerline icons (and much more) built in: https://github.com/ryanoasis/nerd-fonts/tree/master/patched-... which is very handy for emacs/vim modelines.