As I understand it, when you fill a stress track related to a characteristic group, you gain cumulative +1 resistance, which reduces incoming stress (as long as that track remains full) from related tests. As well, you gain a trauma related to that characteristic group, which adds 1 negative die to related tests.

In short, it adds a stress die, but reduces incoming stress. The only purpose to this is that it gives a chance to cancel out a positive die without the risk of taking actual damage.

Is this accurate? If so, what is the benefit of it just cancels itself out? This seems underwhelming considering you just suffered a massive detrimental effect.

You don't gain the Trauma until you "rest"/assess your damage, at which point the Stress (and Resistance) goes away. So yea, you are misunderstanding a lot.