Thought experiment here, just for fun:
You are at your favorite baseball diamond. You are holding a piece of plexiglas in front of you, and you have a wax crayon in your dominant hand. You will be drawing lines on it based on what you see on the other side of it. Or you can draw on your TV screen, but I can't be responsible for the long-term effects on your relationship with your significant other. Ready?
You are sitting in the stands between home plate and first base. The pitcher throws the ball to the catcher. You follow its progress and draw the line on the plexiglas as you see it go. The line goes from...? Correct, a long line, going from your right to your left, dropping down a bit with gravity.
Through the miracle of time travel, you are now on the other side of the diamond, between third base and home. The pitcher throws the same pitch. Where do you draw your line? Correct-- the ball appears to go a long way from left to right, again dropping down a bit with gravity. Same pathway, different vantage point, different crayon tracing on the plexiglas.
OK, now you are sitting right behind the pitcher. Same pitch, coming straight at you. When you trace its path on your plexiglas, what does the line look like? NO right-to-left, because it's coming straight towards you, so you draw a very short line from the center of the plexiglas, where you see the pitch leaving the pitcher's hand, dropping a little bit to account for the gravity as it settles into the pitcher's mitt.
Last, you could even pretend you were in the grass between the pitcher's mound and the plate, and seeing the ball coming by. Your line would rise as the ball approached you, and then fall as it went past you.
In EKG-land, the electrical impulse traveling through the heart's anatomical conduction system is that ball. The different lines you have drawn on your plexiglas are the picture of that impulse as seen from many different vantage points. Why bother? Because it's important to see the path clearly as it moves about the heart. Some leads "see" the P wave better than others. Other leads, because of their vantage point, show the QRS upright or downgoing. Depending on where the lead is, it might show derangements in conduction better, like a bundle branch that's damaged (bundle branch block). So the derangements in some leads specifically tell you about how the conduction goes through specific areas of the myocardium, with the understanding that abnormal conduction (an abnormal line) means something wrong in the muscle.
You might not see the P wave in one lead and wonder where it is. There's (probably) still conduction from the SA node to the AV node, but you're looking from behind the plate, not from first base, so you don't see its movement so well.
I hope that helps some. I love baseball. Go Red Sox! (We made more than $3.3M for the Jimmy Fund last week on the telethon!)