Originally, poetry was a purely oral medium. The earliest forms of written poetry do not distinguish poetry from prose by any visual techniques. Archaic Greek poetry, for example, was written in scriptio continua, with no breaks between words or lines. It wasn`t until the Hellenistic period, probably due to Greek poetry being taught in the grammatical curriculum to non-native speakers, that the features we now think of as standard were used. In English poetry, by the Renaissance, there was a tradition of breaks at the ends of `lines`, i.e. of repeated metrical units, and capitalization of the first word of each line became common. Following these conventions in the 2oth and 21st centuries indicates usually a neo-traditional or neo-formal stance.
In free verse, line breaks are used to indicate where a reader will pause, as will other forms of spacing. `Concrete` or `visual ` poets work to make poetry a primarily written rather than oral medium, and use spatial indicators (character size, underline, font, etc.) to signify elements of meaning rather than sound.