Power Supply for L293D
Driving motors is a dirty business, especially when you're driving them with PWM - you're switching relatively high currents on &amp; off at kHz, which generates a lot of noise, and is fairly demanding of the power-supply; and that's when everything is going well!  If a motor stalls due to a heavy load, some motors can draw even more than their usual current.  Motors also put inductive spikes back into the power supply, which is why you try to deal with as much of that crap with the fly-back diodes, but diodes aren't perfect.

This is about the worst kind of power-supply you want to use to also power the MCU that's controlling your motors!  If your MCU locks up because Vcc sags too low the MCU stops working completely, or if the AVcc or Vref (reference voltage needed by the ADC) inputs to the ADC are noisy then your analog readings are less accurate, all of which can be annoying at best, and dangerous to human life &amp; damaging to property at worst.  This is why a separate power supply is used for the MCU &amp; related digital peripherals, or at the very least a separate voltage regulator.