Where should I add my customer CSS code?

Recent version of WP has this Additional CSS option built in. Personally, I’d use this option to put in additional/custom/extra CSS, so they can be managed in one place. Use CSS comments for each block of code so that I know what code is doing what.

Some themes/plugins has its own Custom CSS option too. I suppose this had been done long before WP has this option built in, they still keep having it so that users don’t get confused.