Why do we use +0V and +5V instead of +0V and +1V in DLD, when it is always '0' and '1' ?

In DLD, the circuits of logic gates (embedded in IC's) are operated with +5 Volts input. That is why we refer to +5 V for these logic inputs. It is considered as binary 1 when the +5V are applied to the logic gate, and binary 0 when 0 V are applied to the logic gate.