Take two poems as examples:
April by Ezra Pound:
Three spirits came to me
And drew me apart
To where the olive boughs
Lay stripped upon the ground:
Pale carnage beneath bright might.
and Veni, Creator Spiritus by John Dryden:
Creator Spirit, by whose aid
The world’s foundations first were laid,
Come, visit ev’ry pious mind;
Come, pour thy joys on human kind;
From sin, and sorrow set us free;
And make thy temples worthy Thee.
O, Source of uncreated Light,
The Father’s promis’d Paraclete!
Thrice Holy Fount, thrice Holy Fire,
Our hearts with heav’nly love inspire;
Come, and thy Sacred Unction bring
To sanctify us, while we sing!
Those are just the first two stanzas of Dryden’s longer poem.
Ezra Pound’s poem has irregular metre, three feet in the first line, two and a half in the second, three in the third and fourth, and three and a half in the fifth. The poem also does not have a rhyme scheme.
Dryden’s poem has consistently four feet per line with a rhyme scheme aa bb cc with an irregular rhyme in the first two lines of the second stanza.
I think that the rules create a pattern which sounds in the ear and registers in the mind and give the poetry its beauty. The first modernist example by Ezra Pound lacks those rules and becomes closer to prose than poetry and loses some of its power to move the reader or listener. This could also be applicable to music which relies heavily on dissonance rather than harmony.