Yeah, and that's a rumor that I'm trying to squash out from this subreddit.
The idea that anime is created as a form of advertisement for the source material is just a ridiculous concept. I have no idea of it's origins or who started the rumor. The idea (kind of?) makes sense at first glance, but think about it, actually think.
Why in the world would a producer want to boost the sales of a light novel/manga? This is a business, the producer is in it for the money. So what exactly would boosting the sales of the source material do for the producer? The producer makes absolutely none of that money. That's a terrible business model that makes absolutely zero sense for a lot of the people involved in the production of the anime. A bunch of people throw money into the anime, different companies, etc, and again, they absolutely want the anime to do well. That's all they're in it for, to make profit from the anime, because again, they're not making any money from the source material. Anime is just like manga or a light novel or whatever, it's a different medium to make revenue. Anime makes plenty of money through broadcasting rights, merch, blu ray sales (which by the way the owner of the manga makes no money off of). So how exactly does this make sense? Why would the these producers want the source material to do well if they receive none of that profit? As I said earlier, anime is a different form of revenue, and it's a very good one too. Anime makes a LOT more money than manga ever could, simply because it's a lot more global and widespread, and a lot more easy to monetize.
I often hear on this sub that "anime is made simply to promote the source material. That's all." so I beg the question. What about anime that has no source material? Cowboy Bebop, Samurai Champloo, Code Geass? What source material are they boosting? And also, why would shows like Naruto exist? Naruto has 400+ episodes. Wouldn't the source material be "promoted" enough? Why are they completely adapting the entire series, eliminating the need to purchase the source material?
In fact, why would a season 2 exist at all for any anime? The concept of a season 2 from a business standpoint is to milk the original fanbase through merch, blu ray, etc. Season 2's hardly attract more viewers, they're directed towards the same audience that followed season 1. If anime existed to promote the source material, a season 2 would be completely unnecessary.
I have a couple theories as to how this rumor started, because it makes no sense from a business standpoint. I'm guessing this rumor was created to explain shows that ended on a cliffhanger, and fans wanted to provide a reasonable explanation as to why it wasn't continued.. but it was probably ended like this so that if the studio wanted to, they COULD continue the series.
Or maybe the idea that "anime exists only to promote the source material" stems from the original author's reasoning himself. If the original author doesn't agree, the studio CANNOT adapt their material, so maybe people think that the author only wants their material adapted only to raise profits for himself... but again, that's wrong. The author makes a ton of money from allowing his material to be adapted, and think of how honorable the idea of his material being adapted would be. I'm sure the mangaka would be stoked. In fact the only reason they'd disagree is because they don't agree with the direction of the anime, or they don't like the studio..
Anyways, the point is, no, anime is not created to promote the source material. That's a terrible business model and if that statement was true anime wouldn't exist because it would generate no money.