I have observed that WiFi routers with 2 antennas have a better coverage than a router with a single antenna. How does that work out?

I can clearly make out that the second antenna is definitely not a repeater kind of thing, or is it?

Absolutely related: How does having more antenna help in a networking router

The answer in the question sited above speaks of Noise cancellation. I can correlate that to Efficiency in terms of reception quality, packet loss, etc, but not to the signal strength.