What disclaimer means is that if you inherit under a will, no one can force you to actually accept the inheritance. Now, why wouldn’t you want and accept an inheritance? But there are situations where it does exist.
For example, in one particular case, a deadbeat father had not paid child maintenance for many, many years. When his own father in fact died, and the deadbeat was about to inherit a substantial sum of money, he learned that his wife and his children were about to attach on to the inheritance. He accordingly disclaimed the interest in the inheritance just to further punish the wife and children and that right to disclaim was upheld by the courts.
Another benefit of disclaimer can be the introduction of the doctrine of acceleration. For example, if one person benefits for their lifetime and it is to be ultimately turned over to their essential beneficiaries upon their death, they can refuse the inheritance which will accelerate the inheritance of the residual beneficiaries. That can sometimes be advantageous to all parties.