When you first advertise for a product its sole purpose is to make it familiar, this will bring customers in and you have a crowd interested.
But when you make it big enough to have everyone know your name, why bother to spend millions on ads still ?
Because you want to
1) Create a still image in everyone’s head, make them always have you in the back of their minds, this will make them always reach for your product on the shells.
2) People are forgetful, a little reminder will go a long way.
3) If you don’t advertise your competitors will make them easily achieve point 1 and 2.