It allows you to type /sethome <customnamehere> at 20 unique locations throughout the server for your access. I use it to save nice plot builds that I may make. It will also be useful for beaconing if they add it back.
It is in the name... This perk allows you to set 20 homes, where as a normal player would have fewer number of homes. Really useful if you have many plots, or friends plots you want quick access to or for beaconing if they add it back.
Without the /sethome perk, you can only set where people go to your plot. Normal players and those without /sethome can't set any homes anywhere. Note that the /sethome perk is only for in the plotworld.