Popular books use birth order for self-discovery, relationship tips, business advice, and parenting guidance, however, when scientist scrutinized the data, they found no convincing findings that linked birth order to personality or behavior. I believe our common perception that birth order matters is due to the confirmation bias. I think it is so popular because when people first hear about it they immediately look to their own family and other families they know and try to match the birth order characteristics with the oldest, middle, and youngest child in that family. More times than not they are able to find at least one characteristic that matches and so they believe the theory.
Correlation versus causation is something to be aware of when researching the birth order effect. There are other factors that come into play when determining the reason for one's personality traits. One article I read discussed how birth order is linked to family size. There are many reasons family size could affect our personalities, as family size is associated with important social factors. These include ethnicity, education, and wealth.
Another principle of scientific thinking to keep in mind is replicability. Many studies fail to show significant effects of birth order and cannot be replicated.