Cases of Debt & Bankruptcy
If the deceased is carrying creditors, the inheritance may not be worth the consequences the estate brings along with it. Or, on the other hand, it could be that the beneficiary named in the will has such a massive debt load that any estate passing to him or her would instantly vanish to cover the debt. In order to keep that property in the family, the heir could disclaim and step out of the line of inheritance altogether.
In fact, this is why disclaimers were originally invented. Just remember, you can't benefit in any way from the estate you're disclaiming, and you can't change your mind later. Even simple mistakes, like depositing an interest check in your personal account, could render your disclaimer invalid.