Would be very useful for vector layers where you ink your animation!

Here is a quote from inkscape's manual:

http://booki.flossmanuals.net/inkscape/_draft/_v/1.0/calligraphy-tool/

Mass

Mass affects how quickly the stroke follows the cursor. A heavier mass makes the stroke slower and increases smoothness or regularity of the stroke. Values can be set from 0.0 to 1.0. When set to 0.0 the path just follows the mouse as normal. When set to 1.0, the drawing of the stroke is very slow.

Having the ability to "slow down" your stroke makes it much easier to create strokes with good curves from a variety of different angles. Inking without it tends to be a trial and error process.