Because a Will is not just to provide for how your assets should be distributed after death. It also sets out important arrangements you may have made for your child who may be below 21 years of age at the time of your passing. You can name a Guardian who will look after your child and an Executor and Trustee who will look after the money for your child until he or she turns 21.
If your child is born out of wedlock, you must make a Will in order for your assets to be gifted to your child. Otherwise, if you die without a Will, your child will not get anything because the law does not recognize your child as your “legal” child under the law when it comes to the distribution of assets under the law of intestacy.