"...should be a statement of such merit about a product or service that it is worthy of continuous repetition in advertising, is worthwhile for the public to remember, and is phrased in such a way that the public is likely to remember it."

The purpose of the strapline (slogan, claim, endline, signature, etc. - click here to see slogan nomenclature) is to leave the key brand message in the mind of the target. It is the sign-off that accompanies the logo. It says "If you get nothing else from this ad, get this..!"