The function of a surface mount door closer, especially one with a capacity of 30-60kg, is to provide controlled closing of the door. This is done by using a spring mechanism that automatically closes the door after it has been opened, thereby helping to regulate the speed and force of the door closing. It ensures that the door closes securely and reduces the chance of it slamming shut, which can cause damage or create a safety hazard. Additionally, door closers may provide features like adjustable closing speeds and latching action, making them versatile in different environments.