One of my favorite websites to visit is this varied and thoughtful "Poetry and Mathematics" collection of postings by Marian Christie.
Throughout history, people who write poems have often been aided by constraints. When we sit down to write, writing the words that first occur to us -- then shaping the word into extended meanings but following a pattern of rhythm or rhyme or word-count . . . or . . . . For many poets the sonnet, for example, has been a poetic structure that shapes thoughts into special arrangements of words.
In long-ago days, when print and screen versions of poems were not easily available, rhyme schemes were an important aid -- helping one's memory to keep a poem in one's head. Now, aided by widely available print and online visibility, poetry has moved into new forms -- including a variety of visual arrangements.