You can do original hem and still cut off the extra if you don't want the bulk, but do want the original seam. In Shiny's case, as she mentioned, they have to cut off all that excess because it's so much.
The main advantage to keeping the original hem is that you get the original stitching at the bottom in the same color as all the other seams, as ChristineB says. I always feel it looks better that way. When you just turn up the bottom and lose that seam, it just doesn't look right.
Doing "original hem" is incredibly easy. I do it myself, and I don't even have a sewing machine. I do it by hand, using tiny stitches, and I have never had a problem, and you can't even tell I've done it by hand. And believe me - I am NOT a whiz at sewing or any type of crafts at all. It is ridiculously easy to do, and I think the only reason tailors charge so much is that most people don't understand how easy it is to do. It's even easier for a tailor, because they do in the machine.
As I said, you can do the "original hem" technique and cut off the extra if you are taking the length up a lot. You still keep the original seam, but you can't let the hem down again (because you have cut off the extra.) Or, you can do the "original hem" technique and leave the extra. That's what I do, because I usually only take up about an inch or so. I actually learned the technique from that same site that ChristineB posted! It has saved me so much money doing it myself. (Although I would gladly pay someone else to do it, for a reasonable fee.)
I love the way the original hem looks. It is also called "European hem" in some places. Where I live they charge $25 to do it, which boggles my mind!