He doesn't have to worry about fixing his hair or bandana or touching his face all over just before he serves. He doesn't have to worry about how he looks each time before serving, because he knows he always looks good.
Federer had to adjust his hair and bandana every now and then. Nadal has to adjust his hair, touch all over his face, shoulders and shorts. Djokovic doesn't need to do all that because his head is free of head gear and he just focuses on his serve and the point. His head doesn't sweat much maybe because he doesn't wear a hat or bandana that will only make the head feel hot.
Without a head gear, Djokovic's head feels free and more aerodynamic as well.
Federer had to adjust his hair and bandana every now and then. Nadal has to adjust his hair, touch all over his face, shoulders and shorts. Djokovic doesn't need to do all that because his head is free of head gear and he just focuses on his serve and the point. His head doesn't sweat much maybe because he doesn't wear a hat or bandana that will only make the head feel hot.
Without a head gear, Djokovic's head feels free and more aerodynamic as well.