It also implies that it's not down to you, but to some sort of chance.

Outcomes of our actions are influenced by our actions, and are also influenced by external forces - often ones we don't know about in advance. Believing that fact can plausibly have a negative impact on one's motivation and performance. But there is also a positive aspect to having your locus of control moved outward, other than the whole accurate beliefs thing: a poor outcome is less indicative of poor performance, and a very good outcome doesn't have to reflect an outlier performance one is doomed to regressed away from, but merely an outlier event, unrelated to your skill level - a skill level which surely isn't waning to the detriment of your performance in status-granting contests, no sir.