Most commercial mass-produced bread is made using the Chorleywood process, which is heavily industrialized, using emulsifiers, enrichment and a highly controlled environment to quickly and cheaply bake bread. It produces the light and soft types of bread most people in the US, UK and Canada (maybe Australia and New Zealand too) are most familiar with.
'Artisan' bread is made using more traditional methods, with at least some of it being done by hand, and time being given for proofing and flavor development. This type of bread often has a good deal of 'oven spring', where the heat of the oven causes a rapid expansion, so bakers slash the bread to allow it room. Without the slash the bread won't be able to expand and will be too tight a texture, or the skin may rupture and ruin the shape of the loaf.
Chorleywood bread can be just about any shape, and doesn't need slashing as it gets very limited oven spring, so the appearance is a matter of marketing. A split top gives the loaf a 'homemade' look to some, and distinguishes those loaves from others on the shelf.
As for why you'd buy one or the other it's a choice between quality, price, convenience and personal preference. I prefer artisan bread (I bake my own regularly) to chorleywood process because I prefer the taste and texture, however it does cost a lot more. I do buy you standard chorleywood stuff as well because it's convenient at times, and I'm not made of money.