Even though your will is meant to be a force for good, preventing potential arguments and disputes over who has right to what in your estate. there are situations where a will may still cause contention among family members. Challenging a will is a crucial step for an heir or potential heir who thinks that he or she is not receiving something that he or she should rightfully get. It can also be important to protect families from a poorly-crafted will that may or may not have been properly updated by the individual.
For example, imagine that someone who has a recent will actually completed another will -- with very different instructions and provisions -- earlier in his or her life. That will could still be legitimate, and thus nullify the more recent will, if certain circumstances are met.
Along the same lines, if the heirs to an estate believe that their deceased family member was manipulated or that someone forged the will in some way, this could provide the legal grounds to challenge a will.
There are even cases where families become estranged over the years. During this time, an older family member may write people out of their will -- but then as their time starts running out, they reconnect with their estranged family members and re-establish their relationship. This could lead to the challenge of a will that was never updated (and still reflects that those potential heirs that were written out of the will get nothing).
Source: FindLaw, "Reasons to Challenge a Will," Accessed March 27, 2015