In poetry, why do rhyme schemes, sonnets, onance, consonance....etc exist?

In structured poetry such as sonnets there is usually a rhyme scheme and you are limited to 14 lines with a total of 140 syllables. This is a discipline and keeps the writer to say what he or she wants to say in a very confined space. This means no wasted space, in other words it makes the writer concentrate and think on what they are doing. In free verse there is still a need for some discipline otherwise we end up with a novel! onance that is the way words work together and SOUND together is very important whether it is in structured poetry or free verse. The painter Pico painted in what some would say is a wild style in later years but in his youth he painted in the clical disciplined style, which give the skills that can be used later as a style is formed.