Another reason why establishing paternity matters - we have a client who came to us to open an estate for her deceased father.
She was worried that there would be a challenge to her status as the sole beneficiary because her father was not on her birth certificate. However, dad held her out as his child her entire life. She was raised by dad's mother. The entire family knows she is his only child, but one of dad's brothers is greedy and wanted to deny her as the child in order to cut her out of the estate.
Luckily for her, dad put her down as the child beneficiary on his life insurance policy, and she was also listed as his loving daughter and the light of his life in his obituary. Poor uncle cannot defeat this extrinsic evidence. That being said, get your affairs in order! Let us know if we can help you with a paternity issue.