Why and what kind of breads should be scored?
A properly formed loaf, before it is baked, has a network of gluten strands stretched tightly around its perimeter.  This network is in tension, helping maintain the shape of the loaf. 

There are several reasons to score the loaf:


The tension in the outer network of gluten strands limits the ability of the loaf to expand, especially in the "oven spring" or final excited burst of activity that the yeast go through as the temperature of the dough rises at the beginning of baking.  The score creates a slit in the network, allowing the loaf to expand, permitting more oven spring to take place.
The scores provide a weak point (due to the cut in that gluten net), controlling where expansion takes place oven spring proceeds.  This may prevent cracks that would otherwise be unattractive.
Furthermore, the score allows aesthetically pleasing shapes or patterns to created on the loaf, as the the dough exposed during oven spring will have a slightly different texture (it isn't surrounded by the in-tension gluten network, and will have been baked slightly less as it comes to the surface of the loaf during the oven spring).  This means it will tend to have a different (often lighter) color than the main crust where the in-tension network was in place.  For example:



(credit on photo)

Another example, taken farther for a more artistic statement:



You can see the attractive pattern on the loaf from the scores (which are also functional in the first sense)&mdash;the look like a leaf.  (Credit on photo).


Scoring is more important and effective in free form loaves which will have good oven spring.