In her note for the soundtrack CD, Nora Ephron, the director of Sleepless in Seattle, writes that "We had made a movie in which the words were as important as the pictures, and we wanted songs in which the words were as important as the music."
Song quotations can be used in a variety of ways, in both diegetic and nondiegetic contexts: as style topics, as references to time or place, to establish mood or pace—in other words, in all the same ways as original instrumental underscore. Beyond that, quotations involving a song's title or lyrics can make obvious references that are the verbal analogue to mickey-mousing—such as "In the Wee Small Hours of the Night" when Annie goes downstairs, unable to sleep beside a snoring Walter. Instrumental quotations, however, depend on a fund of cultural knowledge in viewers.
In this film, Ephron and her music director make sure that we always "get the connection": as she puts it using as an example "Bye Bye Blackbird," which eventually acquires considerable importance in the underscore as well: "'No one here can love or understand me' [Joe] Cocker sings, as Tom Hanks and Meg Ryan sit alone in the moonlight 3000 miles apart from one another; and as we cut back and forth between them, the music becomes the link, almost as if the song is going through both their minds."