Best guess: each antenna is optimized for a different carrier frequency and splitting traffic between antennae allows the designer to use multiple, lower-cost parts on each data stream rather than a single, higher-cost part that can handle one antenna dealing with all the traffic.

Multiple antennae carrying the same frequency can make a difference, but consumer electronics where the end user has control over the angle of the Antennas likely isn't precise enough to make use of the potential benefits.

If the antennae were very precisely positioned and had very precise phase offset, the full array could be used to have very tight control over polarization...which really doesn't matter in a home wifi environment.

OR! It's just for looks.