Advertise comes from the Old French verb advertir, to notice, and the even older Latin advertere, to turn toward. People advertise to get other people to notice them and to turn toward their products and services. That's pretty obvious, I know, but what may not be evident is how advertising works. There are instances where a single ad or a short flight of announcements will do the job. Promoters for such events need only to tell fans where and when to buy tickets.
Fan loyalty does the rest.
But most other products and services require advertising and promotion on a continuing basis. Advertising increases sales. Early on, I wondered about that too, until someone pointed out a simple truth. The sun comes up fresh every morning. People who want other people to notice have to work at it all the time.