Later Fermentation Additions?

In one of my brewing books, the author and owner of Dogfish Head will occasionally ferment a strong beer to a somewhat low gravity (~1.030) and then add more sugars to the fermenter in the form of syrups or fruit.

1. Why is it done this way rather than all at once? Flavor? Will this new sugar ferment out all the way?

2. How do you calculate the alcohol % when you are adding x amount of sugar to an already partially fermented batch of beer?

3. Will a barley wine style yeast be stronger when used in this matter, because it will eat the original sugars and then grow and then eat the other sugars that are added later? So I essence the beer can get to a very high abv% when this procedure is followed. Is there a limit or can you just keep adding sugar to an already 10% beer and it will ferment to 12% or 15% or 20% because the yeast has HULKified itself? Please explain…

1 - as to not stress the yeast with so much sugar off the bat
2 - keep track of everything you have add up the points and minus the FG
3 - it wont stress it out as fast so you can generally get it to ferment more, wlp099 can get beer up to and over the 20% mark using this method.