TPC, Transmit Power Control, is actually a feature of 802.11h along with DFS (Dynamic Frequency Selection).
TPC, Transmit Power Control, is actually a feature of 802.11h along with DFS (Dynamic Frequency Selection) by which the access point can define local rules for maximum transmit power.
There are many reasons why this would be used. One could be that the administrator wants to set another set of rules than the regulatory domain maximum because of more specific local rules or environment.
Another could be that the administrator knows it is a very dense Wi-Fi deployment with a intense coverage : therfore APs set themselves to a lower transmit power and TPC is a static way to force clients to also lower their power and therefore lower their coverage so that they do not disturb neighbor clients/APs that are on the same channel.