Ads serve as reminders I suppose.
Like a new game announcement trailer is technically an advertisement designed for you to want the game. But often the game was already confirmed to be bought the second it got announced for me. Many trailers change people's minds though. Like the new God of War (2018), it had massive fanfare and got many more people to buy it than the older games. If there was no trailer showing off the game or any info then people probably wouldn't buy it in as many droves. For me, I kind of got turned off by it and decided to not buy it until it is cheap.