That is indeed a great question Ace, and I think Bob has given one of the best explanations that I have yet to see. As he stated, dive planning is all about recognizing your limitations on dry land before you ever splash, and designing your dive around those limitations. Divers who simply jump in with no clue as to how much gas they are carrying or how long that gas supply will last at their targeted depth, they are typically the ones that see something cool and get lured in. Just like the sailors of old who were said to have answered the call of the sirens, these divers may soon find themselves in a situation they are not prepared to deal with. Having a dive plan allows you to look ahead at possible emergency situations and formulate a plan for recognizing the early signs of impending disaster, as well as how to respond safely. Your dive plan, if correctly laid out and properly executed, will ensure that you return to the surface safely without the need for a rescue. If I see something cool, I tend to make a note of it on my slate and then plan my next dive to explore in more detail, as opposed to deviatng from my plan. Finally, a good dive plan can be a godsend for anyone having to do a recovery; a very close friend drowned back in February, and given the system he was in the team was not looking forward to the task ahead of them. Then they found his survey notes from previous dives and were able to piece together his last dive plan which took them straight to where the body was. That may seem a bit grisley, but that individual would not have wanted anyone risking their own lives to remove his lifeless corpse from that or any other system.