Antennas don't add power. They can only redirect existing power. Higher "gain" antennas have a narrower beam. Inside that narrow beam, better signal. Outside that narrow beam, worse signal because you stole ability to receive from that spot and moved it to the other spot.

Easy visual. Flash light vs laser pointer. Aim a flashlight and it hits a specific spot. Replace the flashlight with a laser pointer on the exact same vector, it might still hit that spot or it might not. The intensity of the light will be brighter in the spots it does hit though.