When drawing from a photograph, one copies a two-dimensional image onto a two-dimensional surface. It is like transcribing a text from one source to another. You may take liberties with the transcription, but there is no need for interpretation. Meanwhile, drawing from life requires converting three-dimensional reality to a two-dimensional surface. It is like translating a text from one language to another. Interpretation is at the heart of the task.
We already know that one of the pitfalls to faithfully representing forms that we see in nature is that we usually already know something about those forms. We have a tendency to emphasize that knowledge to the detriment of the drawing. A successful interpretation is dependent upon our focusing on what we see to the exclusion of what we know. Only rarely will knowledge of a form prove more useful than deleterious. To move beyond what we know, we must reduce the third dimension into shape, often with proportions far removed from what we know about the physical object.
Consider the constellations. We perceive the stars, despite their differing levels of brightness, as equidistant points adhering to the dome of the sky. They appear relatively flat despite the thousands of lightyears of depth between the points we are connecting. In a similar fashion, we imagine the picture plane, with our paper as its analog, and connect the points of the drawing.
Sighting is one mechanism by which we make these connections. It greatly simplifies the translation of the three-dimensional to the two-dimensional.
Of necessity, this work is tentative -- not in the pejorative sense as we might use it to describe a "sketchy" line where a confident line is wanted, but in the sense that we are working our way toward the "truth" of the drawing by suspending judgement until the drawing is concretized. Techniques for sighting are covered in class. In the meantime, come to grips with the idea that your drawings will not be "perfect" by reading the post "Process over Product."