A scratch vocal is a vocal performance that a singer records to provide a reference track that music producers and audio engineers can use as they craft other pieces of the recorded song.
It is primarily for timing purposes. It may also provide an idea of how the vocalist may ultimately perform the vocal, making it easier to add matching musical embellishments. A scratch vocal is often recorded quickly, and the singer may voice-mark the song rather than hit more difficult notes.
Most of the time, the singer ultimately re-records the vocal performance after production is complete, however there are a number of exceptions to this rule such as in the case of The Piña Colada Song by Rupert Holmes where the re-recording lacked the desired energy and spontaneity - or in the reverse case of Superstar by The Carpenters where the scratch was so well performed that a re-record was deemed unnecessary.
Scratch vocals are also often used in the production of feature-length animated films to bring storyboards to life as "animatics," in which storyboard frames are synced to the relevant dialogue, together with a rough soundtrack generated on a synthesizer. Scratch vocals may be obtained from professional voice actors (who may or may not be well-established in the voice-over community but are generally unknown to the general public) or from anyone around the studio willing to chip in a line or two (as well as friends or family members). For lead roles, scratch vocals are nearly always replaced in the final cut by vocal tracks recorded by bankable stars or experienced character actors. However, in the rush to meet deadlines, if the scratch vocals for a minor role are good enough, the director may skip auditions and simply use the scratch vocalist in the role. This is how many animation studio employees (and their friends and family members) end up with minor credits as cast members on their studio's products.