The simplified schematic of the buffer inputs "just ending" is supposed to be intuitive to indicate the decoupling of noise from the load to the input as well as source supply.
When you drive high currents with logic you risk getting supply noise back into your logic and this degrades the reliability of your signals which can cause glitches or false triggering. This is especially true of any reactive loads and motor loads with commutation noise.
The beauty of this design is that the logic and drivers use separate supplies. Although they share common ground potentials, careful design (beefy ground tracks and/or ground plane and/or isolated ground distribution can mitigate noise ground-shift false triggering from EMF kickback diode shunt current or other surges. By the way these drivers also have ESD protection and diode clamps built-in for inductive loads. Designers are wise to use common mode chokes on stepper motor loads to reduce EMI.
The complementary driver is often called a "half-H" or "half bridge" switch. When the load is connected between two such half-H drivers with one side inverted, you can get twice the load swing useful with bi-directional motor control with PWM controlled by the enable signal then you have a full-bridge of full-H driver configuration. Dead-time control to prevent shoot-thru or shorting of the supply to ground via the complementary outputs becomes a critical design factor in high current inductive loads.
TI describes it as follows:
The L293 and L293D are quadruple high-current half-H drivers. The
L293 is designed to provide bidirectional drive currents of up to 1 A
at voltages from 4.5 V to 36 V. The L293D is designed to provide
bidirectional drive currents of up to 600-mA at voltages from 4.5 V to
36 V. Both devices are designed to drive inductive loads such as
relays, solenoids, dc and bipolar stepping motors, as well as other
high-current/high-voltage loads in positive-supply applications.