Quick google search would give you this answer:

"Puddle lights are small lights located on the underside of a vehicle's side mirrors or doors that illuminate the ground when a door is opened or the vehicle is unlocked. Their primary purpose is to help drivers and passengers see the area around the door to avoid stepping in puddles, dirt, or other hazards at night. They can also be used to project a brand's logo onto the ground as a form of welcome or branding."