On hearing the words "Just do it!", you will know there is a Nike product nearby. If it's "Always Coca-Cola", you can be sure someone wants to sell you a refreshing drink.
An advertising slogan is to a brand what eyes are to a person. The slogan helps people understand the brand better by telling them what it wants to sell to its customers. Good slogans leave a message inside people's minds. It's almost certain that every brand has a popular slogan.