They scavenge it from another part of their coverage! So, you increase the effective power in one direction and significantly reduce it in another.!Quite correct MHC but it appears that you are just thinking about the horizontal radiation pattern from antennas such as Yagis
Think about the vertical radiation pattern from an antenna such as a vertically mounted dipole for instance.
There is a lot of wasted energy being transmitted above and below the remote station (the device I want to connect to my router's Wifi).
If I can bring that pattern in and concentrate it's pattern more laterally (but still omnidirectional) then Bob's your Uncle.