A dress form, or body form, is not cheap - but it's one of the best investments you can make if you plan on making clothing with draping elements patterns.
Advantages of draping a pattern on the dress form includes
Fewer alterations as you see the fit of the garment as you make the pattern on the doll.
The design is seen in its full glory and not just various pattern pieces
Bias draping for ex cowl necklines are controlled by you
The fit and style lines are determined by the designers preference
See how your handmade clothes look on a 3D body
You are able to mark an even hemline
You can test out embellishments without sewing them on permanently
Although some might prefer using flat patterns to create their garments, draping patterns on a dress form eliminates a lot of potential problems regarding the fit of a garment. Making patterns on a dress form might not be easy at first, but when one fails, try try again. Practise makes perfect.