[Python] When is plt.show() required to show a plot and when is it not?

Since the following code will show a plot without plt.show(), what is the point of plt.show()?

Please tell me when plt.show() is required as this would allow me to appreciate the intricacy of matplotlib better.

N.B.: I'm using this in Spyder (Anaconda)

import matplotlib.pyplot as plt
plt.subplot(211) # the first subplot in the first figure
plt.plot([1, 2, 3])

Continue reading...