Drawing linearly requires both an understanding and sensitivity to line quality. The topic of line quality is a large one that will concern you for the entirety of your art career. Suffice it to say that how you handle line plays a major role in how your drawings and paintings are read.
Line can be used to create a sculptural sense of 3-dimensionality. A heavier, darker line will advance while a thinner, lighter line will recede into the picture plane.
Line also plays its role in conveying the character, or personality, of both the object and the artist. An old running shoe is generally best rendered with short angular lines that express worn and creased leather. Drawing an Art Nouveau vase, on the other hand, might be better expressed with a graceful, serpentine line.