The first is the sheer sensuous pleasure of drawing, something shared with virtually all other painters. Drawing is simple. Of course it always returns because it is so simple. Nearly all the great visual discoveries could be shown with a pencil. Probably drawing is as near to instinct as can be found in art.
Drawing is a way of remembering.
It is a way of reliving. The process of drawing, not the drawing, is what counts. Drawing is a way of focusing. In a museum, if you wish to examine a Titian, or a Rubens, or a Piero, then your best way is to draw it. Make a copy, even if it is only in your head.