In practise, like when you go for the driving test vision testing, the Snellen chart is tested at a distance of 6 metres because

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It's the distance that matches refractive power of the eye to the size of the middle line of text

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It's the distance that matches refractive power specifically of the lens to the size of the top line of text

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It's the distance that compensates for the decrease in light intensity with distance

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It's the distance that matches refractive power of the eye to the size of the top line of text

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It's just a convenient distance to use

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