The answer from "Osman" has to be some of the most ignorant babble I have ever seen on this site, and I have seen plenty. It is one of those rare instances when I DO put a thumbs down on an answer.
Why *I* use the viewfinder:
1. It is how I have done it for over 30 years.
2. I can keep the camera FAR more steady against my head rather than trying to hold it out at arms length
3. How you ever noticed that everyone who takes a photo using the monitor CANNOT bring the camera up and get a fast, candid type shot? They are always moving the camera around, in and out, up and down staring at that screen trying to get some kind of composition or focus. I can have half a dozen shots taken in the time they are still wasting time trying to get everything looking right in that monitor.
4. Bright sun directly at my back shining onto the camera... no problem with the viewfinder. I almost have to laugh at people struggling to see that silly monitor instead of just using the viewfinder and getting the shot already.
5. The monitor EATS up battery power. I can get at least 10 times the life out of a battery by not using the monitor as compared to people who use it all the time.
On a related note, I also find it absurd to see people who have to "chimp" after every shot. That means they have to look at the photo they just took on the monitor every time. To that *I* ask WHY? If you know what you are doing with exposure and composition and focus, there is no need to do that. I have my camera set to not even display the photo after the shot is taken. I would wager most anything that since you are using the monitor to take your photos, you are also "chimping" with it after every shot. Good luck with that in some kind of fast moving event. You will miss 75% of the shots I will get using the viewfinder and not chimping.
steve