All the problems in the world can be classified into one of two categories:
Ignoring those problems that are SomebodyElsesProblem is a good way to focus my attention on those problems that are mine.
Ignoring those problems that are SomebodyElsesProblem can keep me from becoming entangled in the political, social, and economic aspects of their solutions. Keeping ones nose out of other people's business is often a good idea. Fixing other peoples problems for them may prevent them from learning how to fix problems themselves or prevent them from fixing them the way they want them fixed.
It can be too easy to dismiss problems as being SomebodyElsesProblem. Just because a problem is somebody elses problem doesn't mean it is not mine as well. A problem that doesn't interest or affect me today may do so tomorrow.
Refusing to help others with problems that are SomebodyElsesProblem often leads to resentment and lack of cooperation. People who explicitly say "That's not my problem" are generally viewed negatively. Diplomacy is important.
In a team environment, it is usually beneficial to consider all problems to be "our problems".
However, you can also use a SEP field to make your boss think the project is finished. See DouglasAdams
See also DelimitYourResponsibility