The human body is three dimensional and not flat whereas
clothing is made of flat material called fabric.

By manipulating the pattern and adding darts and/or tucks, it is made
into something three dimensional that fits the body and feels good to wear.

Other than this, darts and tucks are used to make the joints like
knees and elbows bend easier and also to prevent fabric pull when bending.

The pants called DRAWERS utilize this.
It is three dimensional around the knee to the hemline because of darts;
you can feel the garment keep up with the movement of the knee.