This is a question that cropped up just this week, many beginners are confused about drawing boards, are they necessary? Well they do have their advantages for drawing. The drawing board is a support that allows you to draw at an upright angle which is better than drawing flat on a table or desk. The reason is because you can draw without having to keep looking up from your drawing.
When you are measuring with a pencil the measurements are more accurate once transferred to the paper as you are not distorting the angle when your paper is upright or more upright. It helps if you can see your subject over the board so you are not stretching your head over the board, so I position my board at an angle that is comfortable for seeing over the top. I like to prop my drawing board against a desk or another chair in front.