The contact made by your hand on the paper will leave a greasy film. The natural oils in your hand will rub off and leave a fine film over the paper.
This change in texture is very difficult to work on, to draw on and should be avoided at all costs.
I use a piece of tracing paper, I like the smooth feel and the fact that I can still see through it, which I attach to my drawing board, not directly to my drawing.
When drawing always use a hand-guard to protect your paper from smudging and grease.
Hope these snippets of info are being beneficial to discovering a little more of the behind the scenes on how a drawing comes about.
Coming up over the next few weeks will be more simple short tips relevant to pencil choice, blending, getting darks dark, starting point, layering but until then you are very welcome to check out
Short Drawing Tip #1: Paper Choice
Short Drawing Tip #2: Warm-Up