It's known as the 'Water Salute' The spraying of airplanes as they land is a tradition known as the “water salute.” As explained by Wikipedia, it's performed for ceremonial purposes, such as the retirement of an airplane.

When you see water being "pumped" or sprayed onto a plane upon arrival, it is almost always a "Water Cannon Salute." This is a ceremonial mark of respect where two airport fire engines spray high-pressure arcs of water over the taxiing aircraft, creating a celebratory tunnel. In 2026, this tradition is used to honor the retirement of a senior captain, the inaugural flight of a new airline to that airport, or the final flight of a specific aircraft type (such as the last 747 flights). It is also frequently used to welcome home high-profile figures, such as Olympic athletes or heads of state. This tradition originated from "fireboat salutes" in the maritime industry used to welcome ships into port. It is strictly a ceremonial gesture and has nothing to do with cleaning the plane or cooling the engines, although the pilots must briefly disable the aircraft's ventilation system so the "rain" doesn't get sucked into the cabin's air supply.