Not all doctors offices or clinics have frosted glass at the reception desk but the reason some doctors offices, clinics and even dentist offices have the frosted glass at their reception desk has to do with privacy concerns.
At the receptionist desk confidential information is being filled out and signed and even patients talking about confident information on the phone so they have frosted glass to prevent the information from being seen and it also sometimes helps to muffle the sound of the receptionist talking on the phone.
That's the only reason they have the frosted glass on the receptionist desks at some offices.