This added note does two things: 1) Provides a smooth descending leading tone, 2) gives the scale 8 notes. Item 1 is immediately obvious. Item 2 is not apparent until you start soloing with the scale. What the 8 notes do is give you the ability to play chord tones on all downbeats. This is important; it is the most effective way to get long phrases. For example, check out this standard Mixolydian lick:
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