How does having more antenna help in a networking router
Many WiFi devices use (or can use) "diversity antennae".  Because the two antennae are physically separated the signals received at each antenna are out of phase, which can be used to help with noise reduction, particularly multipath interference.

If you have a problem with weak signal strength then diversity may help, but a properly positioned directional antenna (a patch or yagi, usually) will be the most beneficial.  Diversity is most useful when you have a large number of reflections (multipath).