Reasons to turn down the transmit power of your Wi-Fi (2017)

To minimise power consumption their radios typically max at 15mW (12dBm), while access points max at 100mW (20dBm) on 2.4GHz and 200mW (23dBm) on 5GHz. In WiFi the client devices decide which access point they want to associate with and when to switch to next. By lowering the transmit power the connection will break sooner and the client will roam to a better access point.

Source: metis.fi