If you have a substantial estate or a lot of potential heirs, there is a possibility that your last will and testament is going to be contested after you have passed. It is not very common, but it can happen. When it does, families are often ripped apart and the emotional damage can be severe.
It may usually be true that "good fences make good neighbors," but sometimes the fence isn't in the right place. That can lead to a dispute as to where the property line actually is. If the understood boundary has been in place for a long time, under the law it may turn into the actual property line regardless of what the deed says.