Or you can use two different heats, a high temperature to sear and then a low temperature to finish off as is extremely common.
There is literally no reason to do this. Or, if you do want to do this, you should do it the other way around (searing the outside after cooking the inside evenly if you want a more crusty outside). Cooking a steak on a surface hot enough to sear the outside quickly is just wrong every time.
Searing a meat first is a different technique. There is a slight moisture loss traded to result in a very different flavor due to caramelization and the Maillard reaction that cannot be achieved by using a constant temperature.
That's simply not true. The same Maillard reaction occurs at temperatures far far lower than that typically used to "sear" steak. We're really arguing about doing the same thing, but in different ways. As the moisture is evaporated from the meat, is creates a carmelized layer in the meat. That occurs whether you sear the outside at a high temperature (450+ degrees), or do so slower at a lower temperature (like 300-350).
The difference? The slower cooking method results in more gradual moistening layers of meat from outside to inside. Instead of a thin layer of very flavorful (but dry and usually "crispy") outer shell, you get a thicker layer of moist *and* flavorful meat. Do it right and you get that same carmelized flavor nearly all the way through, but without the "crust" effect. The reason is because instead of trying to suddenly evaporate all the water from the surface of the meat all at once, you are gradually and constantly evaporating the moisture from the entire steak over time.
The idea that is in any way the wrong way to cook meat is completely ridiculous.
Want me to start linking to the sheer volume of sites which support what I'm saying? The one link in opposition to me is pretty much the one link that disagrees
. Pretty much ever food expert agrees that searing doesn't seal in juices. It adds to flavor, which certainly matters if you're going to continue cooking something at a lower temperature later (like baking, or putting meat into a sauce), but if you're going to grill a steak anyway you are going to carmelize the outer portions of the meat. When it turns brown, that's what has happened. The same flavor is there whether you sear it or not.
Honestly, the only difference is the texture. Searing makes the outer surface hard and dry. Cooking the whole steak slower and lower will brown the steak, releasing the same flavor, but not dry out any portion of it. That's why it's a superior method to cooking steak. Those who argue otherwise are just plain wrong. Cause I said so! :)