With the raspberry pi 3 model B the board was EMC certified as an end product. That was fine for end-users but it created problems for people wanting to use the Pi as part of a larger product. They essentially had to do the full "intentional radiator" EMC tests again.
To make life easier for people using the Pi as part of a product they decided to certify the 3B+ under the more stringent requirements for RF modules. To meet those requirements a metal can got put over the top of the wifi/bluetooth circuitry.