When do doctors wear scrubs?

1) They are comfortable, especially when worn under a surgical gown.

2) They are the style for hospital (and many other health care) employees. In many places they have become a kind of uniform.

3) The original reason for them was for hygiene. Doctors, usually surgeons, would wear regular clothes when out of the hospital and on rounds in the hospital and then change into nice clean scrubs (done in the hospital laundry and available in the changing rooms) before going into the surgical areas so as not to bring the germs from outside in.