Suppose that you want to know the apparent angular size of the Sun. It might help to draw a sketch (often so!) in order to visualize the problem. The difficulty is that the Earth is around 7,000 km in radius, the Sun 700,000 km in radius, but the distance between the Earth and Sun is around 150,000,000 km. So if on a blackboard you drew the Earth to be 1 cm in radius, the Sun would be 1 m in radius and the blackboard would have to be 20 meters long the accommodate their separation! Fat chance.
Instead, one draws a sketch that is not true-to-life in terms of the
``scales'' (i.e., the relative sizes), but is representative. These
sketches are useful for better picturing things (especially for those
who are visually oriented), but ALWAYS beware of the limitations of
such drawings. If the drawing is not to scale, you can sometimes be
misled. For example, the angular size of the Sun at the Earth is about
; however, a little representative drawing might lead you to
believe the angle should be bigger. You would be misled in thinking so
because the scale of your sketch is not properly represented and will
probably overestimate the size of the angle.