If there is not enough charge to perform the switching action, it will cause a large change in the supply voltage. Voltage variations that are too large can cause digital signal levels to go into indeterminate states and likely cause state machines in digital devices to behave incorrectly. Switching currents flowing through board traces will cause voltage changes, due to parasitic inductance of board traces and for a variety of reasons, bypassing (or decoupling) at the power supply or at the power supply pins of active devices Capacitors are very good practice.