When you are learning to draw you can use a grid to help. If you are trying to copy an image you can draw a grid over the original, then draw the same sized squares on the paper you are drawing on. Finally copy the original onto the drawing by filling in each square. The grid helps you place lines more accurately. If you do the drawing in pen instead you can rub out the pencil lines. It’s really only a device to help learning to draw, or if you want to draw out the basis for a painting when you are using opaque paints like gouache, acrylics or oils. You can also use this for scaling up or down. If the squares are half size on the original the drawing will be twice the size (or vice versa), hope this makes sense.