Gonna play Devil's Advocate for a moment here.

I assume that this isn't actually for nefarious purposes, and is actually just a low-effort way of curbing spambots on their platform. It's likely that the bots are using emulated devices to post from the official app, and this permission might lock up a lot of those bots. Obviously this wouldn't be the best way to combat spambots, but I'm gonna go with Hanlon's Razor on this one.

I know the immediate first thought most people will have is that this is just so Meta can open up another avenue to spy on you. But let's consider for a moment the logistics involved in that. Audio/video data is huge; capturing and parsing it it requires a non-insignificant amount of CPU/battery usage, and transmitting it will use a good bit of bandwidth, both of which would be noticeable by even novice users (since most modern devices these days will show an on-screen indicator whenever certain sensors are being activated, and will tell you what app is using it, so seeing Instagram trigger your mic/camera when you're not using it would be immediately noticed by just about everybody). That would also make this one data stream exponentially more costly to gather and process for Meta than most of their other data streams combined.

Also consider the fact that Meta already has over a million data points on just about every single person on the planet, anyway; what could they stand to gain by monitoring your IRL presence that they haven't already inferred from the other, less-invasive data they've gathered on you? Half of the recordings they'd get would be farts and "oh my god, stop barking, nobody's even at the door", and Meta probably already knows that you have a dog and lactose intolerance.

It's more expensive to produce, it's more likely to be detected, and there's less of a guarantee that you even get any usable data from it at all since they already know just about everything about you already. I really don't see spying as the end goal for this particular action, only because it doesn't seem like a profitable venture.

None of this is to suggest that Meta isn't spying on you. They are. They 100% are spying on each and every one of you. I just don't think the mic/camera are how they're doing it.