It is very usefull as they need to know how you acted at a very young age. Many people keep a kind of growth book of their children, they are very usefull. Other people that have knowledge of your early life are good too, so you can do without your parents, when they are not availeble because of any reason, but it is a lot easier. Plus they can provide some support as well, which I found very usefull.
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"How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?"
Sherlock Holmes in The Sign Of Four (1890), ch. 6