I’m a network professional with a specialty in wireless.
Yeah, beam forming and mimo are the main reasons for antenna diversity. There’s also more radio chains in those units typically, and more radio chains can provide better speeds if you have client devices that can take advantage of the extra radio chains (both sides need to have the same, increased number of radio chains to see an increase).
The antennas are fairly small/thin pieces of wire that are not very long, so the antennas don’t need to look like that, but the quantity is useful.
As someone with a telecommunications background who’s taken apart some cheap routers that look like that: the only caveat I’ll add is that the antennas are only useful if they’re actually connected to anything. From a decently trustworthy brand you’re probably fine, but I’ve seen a few where only one or two of the antenna couplings were connected to anything internally.
