What's the usage of this type of biasing?
They're not being biased into active operation. The rheostat is for calibration. 

The transistors are connected as low-leakage zener diodes so they will clamp errant input voltages that may occur when the user does something idiotic. 

Edit: with regard to your added question, yes there is a difference. The back-to-back series transistors act as back-to-back zeners so they will clamp the voltage at maybe +7V and -7V. The single transistor will clamp at +6/-0.7 or -6/+0.7 depending on the type of transistor and how it is wired.