A person might keep such a secret because for certain events, a deviation from the proper timeline can cause a massive rupture in reality, damaging paradoxes, and general chaos across the web of time.

And for other events, nothing happens and the universe continues to unfold with barely a hiccup. It takes an expert to know which is more likely to occur and so it's best for non-experts to avoid playing Russian Roulette with the timelines.

For the record, yes, I am an expert in temporal mechanics, and temporal intervention in particular.