I put extras in between chapters, because making my pages is hard work, and usually after about 15-20 weeks of non-stop work, I need a few weeks off! I don't put them in the middle of chapters, because it interrupts the flow and sets a precedent of "sometimes when you're excited for a page of this comic, you just get an extra" that might make people stop clicking notifications and affect performance.
I think if they're in-between chapters, so it's established this is "not canon", you can have characters acting like they know they're in a comic. It can be a nice bit of light relief. In extras, I put a lot of things like me, the creator, talking directly to the audience, characters reacting to fanart of themselves, that sort of thing. I also put things that are books that might exist in-universe that can explain more complex, optional lore too.
In the book version, all of these sorts of things go at the back of the book so that the reading experience is smoother.