For example, SP500 9/2018 was 2900. 10/2018 fell to 2700, so I sold all because I sense there was a bear market coming.
What if I was wrong? I pick up my position at 2900, I lost 7%, big deal? not really.
But what if I was right? I might avoid loss of 25% or more.
Sensing market is not pure luck, you have to follow the news and make your own judgment. Jan 18 there was a similar correction I did not sell, but this time is different.