There are several applications for having a legal grandparent DNA test done. Sometimes the paternity of a child is questionable and the grandparents doubt the child is their grandchild and may have trouble putting the grandchild in their will unless paternity is confirmed legally.
They may also have trouble fully bonding with their grandchild if they are uncertain that the child is in fact their biological grandchild. Often times getting a DNA test is the only way to know for certain. This does not have to be done with a legal DNA test as informational tests have the exact same results as a legal test with the only difference being chain of custody of the DNA sample but informational tests will answer peace of mind questions.
The purpose for any legal DNA test, regardless of whether it is paternity, maternity, grandparent, sibling or avuncular is so that it will stand up in court for legal issues. This applies to any kind of DNA test used for legal issues such as custody, child support, child visitation, parental rights, inheritance and estate issues, immigration issues, medical and religious upbringing decisions and such.
A legal grandparent DNA test is an alternate way to determine legal paternity if the father is missing, deceased, incarcerated or simply unwilling to take a DNA test. Since each parent contributes 50% of their DNA to a child having both paternal grandparents do a DNA test with the grandchild in question will be the same as doing a direct paternity test. If the mothers DNA is also included with the test the lab can subtract her markers out of the equation and only study what markers are left for even stronger results of the test.
When people think of paternity testing for custody and visitation rights they usually only think of the biological father or mother. Mostly this is the case but grandparents also want to see their grandchildren and wish for visitation rights as well and often times they are denied this and have to go to court to work this out. Grandparents do not have the same rights as parents and each state has their own laws regarding this but often having a legal grandparent DNA test is required to prove they are the biological grandparents for starters.
It is usually preferred to do a direct paternity or maternity test whenever possible but there are situations when the parents are deceased or unavailable or unwilling to do legal DNA testing and this is where a legal grandparent DNA test would be the next best option.