Reason 1: If the attack is on, it will take long enough to draw. I don't want to waste time racking the slide. I want to neutralize the threat as quickly as possible.
Reason 2: If you keep a round in the chamber and a full magazine, you're carrying one more round than if you didn't keep a round in the chamber, but merely a full magazine. (Was that clear?
)Could that extra round save your life or the life of a friend or relative? Maybe.
I also recommend getting training. I recently went to the PPOH course. Charles taught us to clear jams & double-feeds. His method takes 3-5 seconds--less if you practice it. This is something I couldn't do before the class. This is the type of knowledge that could possibly save my life if I needed it.
Try IDPA. It's a definite skill-builder built into a fun game. I'm consistently the lowest performing shooter at each match.
But am I a better shooter for having attended than I was beforehand? Absolutely. With IDPA, you start your course of fire with a chambered round, a topped-off magazine, and one or two full extra magazines. Since safety is their highest priority, you learn to move about safely with your weapon, then speed starts to develop.