I do a lot of extra content xD In the begining I used to celebrate milestones with exclusive illustrations, i've made 2 Q&A already (my comic's 2 1/2 years old), I post every valentine's, every comemorative date in general if I fell like it xD
Than I have patreon, where I concentrate all the extra content for real. It's basically extra illustrations for now, but i've made short comic strips in the past and a special chapter for valentine's day that explains a bit of past story.
But as a rule, if there's a reason for me to spoil my readers with more piece of content, I'll do it. I love to do that, It's part of the fun of making my comic. But unfortunatelly for free I just post exclusive illustrations with a a bit of context/history behind and just in special ocasions (like the last one was in mother's day). I'm also always talking in the ~description~ area, when I can I drop info that didn't make to the comic yet but it's cool to know, so sometimes it's important to read my rant xD