Why should an area be compared with surrounding values before making it darker?

Correct answer: Because a value is perceived relative to neighbouring values

Pencil shading follows the principle of relative value: a grey patch is judged by its neighbours, not in isolation. The same mark may appear light beside a dark area but dark beside a pale one. Before adding graphite, compare the complete value relationship and ask whether the problem is local or caused by surrounding tones. A dark value is not automatically wrong, and graphite can be built gradually.