In practice
CSAT belongs to a moment: a support ticket, onboarding step or purchase. It doesn't measure the whole customer relationship.
It isn't net promoter score, which asks how likely someone is to recommend you. It also isn't customer effort score, which asks how hard something was.
Why it matters for product builders
Because you ask straight after the event, CSAT can point to a problem close to where it happened. That makes the result easier to act on than a general score.