When you run into a negative outcome, sometimes you’re the only one responsible. But in many cases responsibility for the outcome is shared across multiple people.
In the latter case, even if the negative outcome is not just your responsibility, it’s useful to take full ownership for it.
The first benefit of this approach is that it lets you identify each of your contributions to the negative outcome. This is the first step to fixing your mistakes in the future. You can’t fix what you don’t acknowledge to be broken.
Second, taking full ownership often causes others to own up to their mistakes as well. Many people have an innate sense of fairness that makes them feel bad from having a single person take all the blame. This makes them step forth to rightfully share it.
Independent of whether the second benefit manifests itself or not, the first is enough to justify taking full ownership of negative outcomes.