If you are advertising, don't forget your existing customers. For many products (consumer products like cars or food; business products like SAAS or corporate furniture) selling to the people who have already bought your product is at least as valuable as using it to find a new customer.

For repeat sales (ketchup) it's good to remind people so that when they are at the shop they'll remember to buy it again (assuming they liked it). Same works in business: we bought Dell and the CFO was happy so next year I'll call Dell and see if we can expand our deal.

Also, before social media, it was important that Chrysler specifically advertise to their customers so that they would tell their friends how happy they were with their Chrysler.