What different purposes, exactly? Both serve simple purpose of selecting emoji, there are however certain things that specific to one or the other, like "keep open" checkbox only making a lot of sense in post emoji picker but as much in reactions.
top 5 emoji would be very useful in post emoji picker, especially if it was made dynamic and actually track often/recent used emojis, and so would zoom (see #797)
the sticker picker was added into emoji picker just to unify the two to avoid clogging icons next to submit button and is completely optional. IIRC it's even added there as sorta "extra content" which potentially could be anything.
on top of that unifying features like zoom or search, the popular emojis, tooltips etc would be much easier if it was single component, not two different ones.
i'm a relatively new user and also a dev, and this is confusing af:
i'm getting used to it now, but i kept confusing them. i mean, the one in the subject bar is closer to the main text box than the one beneath it. the buttons below were maybe made big to make this obvious, but the net effect is instead that of them being decorative/confusing bits of graphics, as opposed to buttons you can click.
this is a bit of a different issue, actually. OP is about that emoji picker and emoji reactions (smiley button in the post itself) using different pickers.