Unlike many of his contemporaries (T.S. Elliot, Ezra Pound, et.c), William Carlos Williams wasn't into writing complex poems that take the reader a lot of work to unlock and digest. In fact, he criticized T.S. Elliot and Ezra Pound openly, describing them as "conformists preoccupied with rehashing the literary glories of the past" (source: The Norton Anthology of Modern and Contemporary Poetry, Vol. 1, 2003).
Think of Williams as a kind of poet-rebel. He felt like the poets of his time were going in the wrong direction, and he sought to revolutionize the way poets and readers thought about poetry. He wrote to the editor of Poetry magazine in 1913, saying, "Verse to be alive must have infused into it something of the same order, some tincture of disestablishment, something in the nature of an impalpable revolution, an ethereal reversal" (source: The Norton Anthology of Modern and Contemporary Poetry, Vol. 1, 2003). To put it plainly, Williams felt poets, scholars, and readers needed to rethink poetry and its purpose, and he felt that poetry needed a fresh pair of clothes, ones that reflected current and future fashions.
Perhaps more than anything, he wanted to capture the rhythms of American speech in his poem, because he felt that American poetry was beginning to form an identity and sound of its own, and one that was very different from that of British poetry. This was very exciting for him, because he felt there was something uniquely delicious about the sound of American speech.
Many of Williams's poems seem like they could be paintings. This comes from his desire to capture moments and objects in his poems like photographers capture scenes with their cameras, or like painters capture scenes with their paints and canvases. Williams sought to give notice to the things that often go unnoticed.