The weird-looking title we get prepares us for all the experimental stuff we see in the poem's body. "[i carry your heart with me(i carry it in]" catches our attention, since it looks so not like what you would expect a poem title to look like. More than that, it also provides a good idea of what Cummings's style is all about. We know punctuation is bound to be important, and we know Cummings is also looking to work with the visual presentation of the words he's using. The brackets may also be indicating that this title is the first line of the poem. All in all, his zany title doesn't strike us as so zany by the end of the poem, since we get that there's a purpose to all the punctuation and lowercase type.
From the very beginning, then, Cummings is trying to shock us into having some "fresh eyes" for his poem. The title provides that initial sledgehammer blow to all the "normal" stuff we expect to see. Instead, we get to experience his work in a way that's equally artistic in a visual and literary sense. Punctuation and lowercase type are being used in a way that's visually stimulating, while also furthering the poem's theme of unity. There's a method to all this madness, after all. Come on—it's not like Cummings decided to take a detour in his career with finger-painting and elementary school grammar mistakes.