Poetry, like any art form, expresses the time and culture in which was was created. Inasmuch as we are aware of traditional culture, so the traditional forms or structures help convey the artist's, or in this case, poet's, intent to us; they facilitate the communication.
The effectiveness of the communication is a matter of perception, and the structure or form can go both ways -- it can help convey or obfuscate the poet's meaning. Many who first read a Shakespearean sonnet are daunted; the form and structure actually distance the reader from the intent of the author, but that's merely due to cultural bias; "modern" readers are not used to the structure and language. However, Once these are understood, then the meaning can become clear, and the mode of transmitting the meaning appreciated.
A Modernist form, or lack of structure, can be equally forceful in what it communicates; the structure's lack can convey the author's angst, as if the feelings could not be properly expressed in a tight rhyme scheme.
Both are equally effective and valuable.
Modernist poetry often tries to break exisiting conventions of structure and form, and as this seen as innovative and 'modern' for the time period.
Writers who wrote it desired to make sense of a world in crisis just before and after the First World War, and the changing society, as well as the emergence of science and doubt against religion.