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This is still true of an observational study like the one linked. But the reason longitudinal studies are useful is because you're studying May girl classy and sassy over 50 shirt changes within individuals over multiple points in time, and if there are causal links, having those multiple observations over several time periods let you do more with the data than if you only had a single snapshot in time (a cross sectional study.) For example, in this study, they measured parent reading behavior both at year 1 and at year 3, and they took measurements at years 1, 3, and 5. 
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Some mothers who didn't read at year 1 started reading at year 3, so you can do things like look at changes in those families from the time they May girl classy and sassy over 50 shirt decided to start reading, and if there is a difference from 3-5 that wasn't there from 1-3. You can measure different things at each of these periods and then try to control for other changes happening as well.
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