Artists consider adding contrast or a range of values the fun part of art. First, let’s Draw a value scale from light to dark that will be used to compare the values on your drawing. You want to make sure that you have all the values depicted. The more range you have the more interesting the piece. Without a wide range of tonal value, the drawings will be dull gray images and the details will be lost. Not visually appealing.
Adding a wide of contrast also increases the illusion of depth in your images. 
Notice how all the values go from lightest light to darkest dark. Make sure your finished pieces have all the values represented. You can make a bigger range of values if you want to get more precise with contrast.
The eye drawing uses values to create the illusion of depth. Consider the light source. Where is it coming from? This is a little tricky but if you pay attention to the reflections off the iris and cornea, you can see the source is coming from the front.
(Note the reflection of the eyelashes in the upper part of the eye.)
This drawing was done using white charcoal and #6 pencil. All the values give the piece some dimension and depth.
The eye drawing below was done just using graphite and white charcoal. The whiteness gives the eye a watery appearance.
