[eqvinox]

Considering a single radio, the antennas are for MIMO, which in theory is supposed to multiply bandwidth by number of antennas (i.e. 3 antennas = 3× bandwidth). In practice this is highly reliant on signal propagation characteristics in your rooms, particularly including position (and angle!) of all antennas (both senders and receivers). The second antenna is useful, third maybe, fourth is gonna be quite questionable.

However, they also have more than 1 radio these days, and sharing the antennas on them not exactly beneficial; if you have one 2.4GHz, 5GHz and 6GHz radio each, you might as well optimize the antennas for each radio. And even if it's the same band, separate antennas allow you to have distinct radios cover distinct space with different RF propagation characteristics.

(They're not empty, or at least I haven't found any fake ones yet.)