When you grade patterns in the common linear way, the length of the curves and lines will inevitably change in such a way, that seams will become of a different length. 

Also, a human body is rarely of a standard size. It is quite possible that you or your client are in between standard sizes, and grading a standard pattern in the same direction everywhere doesn't help to fit. Changing a ready standard pattern to fit individual measurements is also a not straightforward task that doesn't guarantee you will succeed.

Sewist redrafts pattern to individual size measurements every time. Since a pattern is based on a described drafting scenario, the program repeats the steps and refers to the individual measurements that you feed into it this time.

To grade a pattern you may simply click the size measurements button in the top panel.

The popover menu will offer you a set of most common sizes, and will let you choose a different size and once you click the height, then the bust, and then the hips measurement. Your pattern will be redrafted instantly.

For example, let us redraft our basic skirt pattern for height = 170 cm, bust girth = 92 cm and hips girth = 100 cm.

You can see how the pattern has changed.