5.33Circle-of-fifths movement appears again

Look at the roots:

A → D → G → C

Each root moves down a fifth / up a fourth.

This is extremely strong harmonic movement.

Add another:

E → A → D → G → C

Make E a dominant chord:

E7 → Am → Dm → G7 → C

Now we're approaching the secondary-dominant material that will become important later.