In general, Angelou uses enjambment as a means of pacing the poem. In the first stanza, it's clear that enjambment allows the speaker to dole out information in bits and pieces, each line broadening the reader's picture of what's going on. Enjambment lengthens thoughts and images (in that these literally take up more space on the page) and also creates momentum in the poem by pulling the reader forward.
While for the most part line breaks tend to coincide with where one might naturally expect to take a breath, the poem occasionally uses them more strategically. One such instance is lines 10-11:
"Come, you may stand upon my
Back and face your distant destiny,"
A more natural line break would be something like: "Come, you may stand upon my back / And face your distant destiny." However, because the line is broken where it is, it encourages the reader not to pause for breath at the end of line 10. The pronoun "my" at the end of the line is inconclusive, pushing the reader to carry their breath over to the next line in order to complete the phrase or thought. By pushing the word "Back" to the beginning of line 11, the poem places a different kind of emphasis on it: it ends up occupying the left margin and being capitalized. Placing "Back" and "face" and "distant destiny" all on the same line allows the poem to actually enact the process being described: the reader must look back before moving forward.
The poem's use of enjambment also, of course, serves to contrast with the use of end-stopped lines. These lines tend to break up the flow of the poem, allowing the reader to land, pause, and take note of where they are.
This happens directly after the previous example. Both lines 12 and 13 ("But seek no [...] place down here.") are end-stopped, lending a sense of finality to the Rock's instruction to humanity to stop hiding. The fact that the line does not carry on forces the reader to pause and consider what they've just read, just as the Rock is asking humankind to pause and consider where it's been, what mistakes it has made, how it may avoid repeating them in the future.
The poem often omits punctuation where readers would naturally pause, creating lines that are only very subtly end-stopped—or that some readers may even argue are actually enjambed. Line 1, for example, contains no punctuation after "Tree," but it's clear from context that the following line begins a new modifying phrase. There is thus an implied pause after "Tree," yet the lack of punctuation also makes sure that this doesn't feel too forced. The reader is still pulled forward, sensing from that lack of punctuation that the thought is incomplete.