A fabricated wafer isn't a shippable product yet. Packaging & test is where raw die become parts that can actually be soldered onto a board and trusted to work. It's the stage where cost per unit gets locked in almost as much as functionality does.
Wafers are diced into individual die, and a first-pass wafer probe test screens out obviously dead die before the expense of packaging is spent on them. Package type (QFN, BGA, flip-chip, and many others) is chosen based on pin count, thermal requirements, and cost, usually decided back at the architecture stage rather than left until here.
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