With the help of constraints, drawing figures to scale happens automatically. If line AB is constrained to a distance of 4 and line CD to a distance of 8, CD will automatically be twice as long as AB.
The same is true of angles. If you want a triangle with angles 30°, 60° and 90° simply constrain two of them (the software will figure out the third).
Of course you may want to purposefully draw a figure that is not to scale. This is where the annotations come in. Using annotations instead of constraint you can set a distance or angle to be anything you want!