If you don't want to be restricted to line work, it is essential to master values to be able to advance in drawing. Values are all the nuances of light that we can imagine. All these shades of intensity enable us to understand forms by the way in which light sculpts everything around us.
Above you can see a gradient. It's simply a matter of passing from pure black to pure white through intermediary shades of grey. Have fun in making the most perfect gradients possible with pencils of varying values.
Creating a portrait from a model is an excellent exercise for improving at various levels. The texture of the skin and the way in which the light sculpts it is ultimately only all the shades of grey positioned in the right places with the right intensity.
I have worked on this portrait so as to retain the shading while eliminating the details, in order to have a more simplified and therefore more accessible picture and to understand the nuances that form it.
The nature of death is another excellent exercise for capturing all the subtleties of the shades of grey. You quickly realise that good depiction of the material and light lies in the subtle shades of grey.
Until you have a good level of experience, it can be difficult to understand the levels of colours. We must learn to observe the light perceived while putting colours to one side. As you can see from the following example, it's easy to transform a colour image to a black and white one digitally, and thereby to see only the values and their roles in the construction of the painting, whether for models, textures, contrasts or composition, and so on.