I don’t understand at all.

Can someone explain why you’d want icons in the monospace font you use for development?

I thought those icons were mainly for building websites. But those are usually triggered with a CSS class, not embedded in source code.

What am I missing?

you can use them on the terminal. there are projects like lsd[1] and vim-devicons[2] that use them.

[1] https://github.com/Peltoche/lsd [2] https://github.com/ryanoasis/vim-devicons