"The Road Not Taken" is a classic poem, regarded by many as the most famous English-language poem of all time, but it is deceptively tricky to interpret.
In your question, you are looking at the final stanza of the poem. The first and the fourth lines of the stanza start with "I" and end with "sigh" and "by," respectively. To understand what this rhyming does, ask yourself, "How do these sounds make me feel?" The "I" sound is open, and it is almost a sigh if only understood as a sound. This sound, the sound of a sigh, goes with the tone of the final stanza. In this stanza, the narrator is reflective, almost sentimental, and is looking back on his/her life. The rhyming of these sounds adds to the sentimental reflection.
Sentimentality is also known to be manipulative. Oftentimes, sentimentalism obscures the facts of an actual event. Indeed, in this stanza, the narrator changes what he said earlier in the poem (that the two paths "had worn... really about the same") and states "I took the one less traveled by, / And that has made all the difference." This is a sentimental trick. He views his actions as having made all the difference, but he actually chose the path almost at random. These sounds ("I") add to this wistful, sentimental shift.