The observer effect in physics refers to how things can change when you observe them. That is, the very act of measurement can change the thing you are measuring.
The same thing happens with Eli, our 10mo old son. If you go into his room to observe him in his crib, and he is even slightly awake, it is pretty much disaster. He hears you, picks up his head, stands up, and starts crying out to you.
When we first looked at baby monitors, I thought video was a bit over the top, but we got one anyway. Now I'm sure it helps both him and us sleep a lot better. With just audio, we would check on him when we heard noises, even if these noises were just due to light sleeping or trying to fall asleep.
But with the video monitor, you can actually tell whether you should go up there or not. You can avoid the baby observer effect altogether.
This works great in the morning. When he is actually waking up for good, he sits up or stands up on his own. But when it is just a passing thing, he stays laying down.
The best use is when he is going down for a nap. It often takes five to ten minutes for him to fall asleep. And he pretty much always cries out right when you put him in the crib, regardless of if he is tired or not. And you're never quite sure if you have the timing just right.
The video monitor tells you whether he is really trying to fall asleep or just upset for getting put in his crib not fully tired. Without the video monitor, you can easily trigger the baby observer effect by going to check for yourself.
We just got back from a vacation where we didn't have the monitor and it was a lot more difficult. We ended up keeping the door cracked slightly and peering through, but that had its own drawbacks. More sounds get through the door in that position, sometimes it cracks open more and makes a sound, and you actually have to get up and go up there.