Lines in art are like arrows! Use actual lines to create movement, point the viewer’s eye to an item of emphasis, underline words that are important, divide or highlight information. Hence, lines can be used as an element of the design or a function for the design.
Designs and photo by Claudia de Almeida for Wired
In design you can also use lines of type for the same task of creating movement or even static energy.
Designs and photo by Claudia de Almeida for Real Simple
Typography overall is made up of lines and your choice of type is based on the message you want to convey.