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Early in primary fermentation, there are volatile compounds that you don't want to keep in your beer. Most often, it's a form of sulfur that ranges from canned corn to egg salad to outright rotten eggs or vegetables. You won't usually detect them yourself, but they're there.
Also, there is a lot more CO2 produced than is needed for carbonation. It's highly doubtful that your fermenter is capable of holding that much pressure. The beer may be 5% alcohol but it takes less than 0.5% for carbonating. The excess carbonation is not pleasant and makes taking samples or pouring a pint very difficult.
So, we usually wait until the end of fermentation and dose with a little bit of sugar to create the desired level of CO2.