Why do commercial helo pilots wear helmets?
The same reason as anyone would wear a helmet - to protect their cranium in circumstances when there is the possibility of damage to that area of the body.

Helicopter operations are often low level, sometimes over dangerous terrain and, unlike fixed wing aircraft, they do not have the option to glide away from an engine failure. Low-level operations may not afford the pilot the altitude required for a successful autorotation landing so there is the possibility that they will be going down hard and fast. 

Taking an example of low-level flight over forestry resulting in a forced landing, this may be survivable, but the pilot will be thrown around quite a bit as the aircraft is buffeted by the trees as it goes down. Some head protection would be important to the survivability of this crash.

There are a few other reasons I could think of apart from the main one above


Some helicopters operate with doors open, up where the birds fly around. A seagul to the head would not make for a happy flight
Noise - helo's can be louder than Fixed wing and the helmets could provide better noise cancelling
Specialist helo operations such as S&amp;R use equipment such as night vision, which could be mounted on a helmet
Habit - ex-military helicopter pilots could just be in the habit of wearing a helmet, and carry this over to civilian operations.