Simple line is also known as the colouring-in style.
Choose this style if you have something complex to describe.
Accessible to the young/adult new reader or a conservative market, it is useful for its clarity. Texture, shading or highlights seldom appear, but grey or thinner lines indicate distant objects/subjects.
Use this style for ‘how-to’ drawings, (e.g. ‘stages of spinning’). All in all, it’s a useful style and works even better with solid slabs of colour. For school workbooks, the line illustrations can be coloured in. No grey areas or shading get in the way. For very young readers, newly literate readers, or a traditional/conservative audience, it’s ideal.