One-piece sleeve is used, for example, in classic men and women shirts.
This particular "by armscye length" pattern is handy when you already have your bodice piece or pattern and now need a sleeve which precisely fits the armhole by length.
Precision of the armscye length. As you know sleeve patterns drafted using instructions from books (and sometimes even ready-made sewing patterns of major publishers) often have the length of the armhole curve different from the length of the armscye of the corresponding bodice. The pattern on this page will have the exact length of the armscye curve as you enter.
Scale independence. The pattern here uses no fixed constants (like "add half an inch") often found in drafting instructions, so you can, for example, use measurements for a doll and you still get a correct pattern.
Precise shape of the cut cone means less unwanted folds (provided armscye on the bodice is shaped correctly as well).
Not all combinations of sleeve inclination and circumference at the end allow to produce a pattern with required armscye length. Currently the app does not notify you properly in these cases, so when you don't get a PDF you might want to tweak these parameters first.