because there are signals that are positive and negative like voice, we modulate them for example in AM or FM, wich carriers are alternanting voltage. In order to operate with negative voltages you must supply the transistors or IC with negative voltage.
Even a computer needs +12, +5, -5 and -12 Volts.
Re: why we need a dual supply in analog and mixed signal cir
with single supply , most of the general purpose opamps will not output 0V with 0V input.when this signal is amplified with further amplifier stages we wil end up with
the high volatge level , also this will affect the effective range of the signal swing.
this is one of the reason we need dual power supply to get the desired voltage swing.