A good intro builds excitement and establishes expectations before jumping into a tune, while a turnaround (typically the last two bars of a chorus) keeps the musical momentum going and leads the listener back to “the top” of the next chorus.
A common pattern is to establish a tonal center, then to move in contrary motion towards the V chord, which creates lots of harmonic tension that is resolved when you play the I chord at the top of the first/next chorus.
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