Answer:
It is because constraints applied automatic by the software (CAD) are supposed to control relationships and geometry between lines, arcs and circles while those manually added are supposed to control the geometry to behave in the manner the user likes the sketch to appear when drawing.
Explanation:
CAD software enables creating sketches using the program by automatic allowing geometric constraints to perform the tasks.Geometry in lines, circles, and other geometric features show collaborating relation that facilitate sketching in the program.For example, two end points appear to make lines remain perpendicular.Other geometric constraints are parallel, and equal.However, the user can manually apply geometric constraints to a sketch to force the geometry in a manner that is suitable to the sketch drawn.That is why a user must manually apply others.
