>>810046
>low effort, low quality advertisement
Not all of them are low quality or low effort, some of them are good and can make certain scenes even better than the manga/LN, with the music, dialog and/or animation. Plus you sometimes get original content in the anime that can be just as good, if not better than the source matterial, maybe the most drastic example being Full Metal Alchemist, with the second half of the anime being original content with it's own original ending, since the manga wasn't finished.
You also keep saying that people have to buy the manga/LN when most /a/nons probably just read it for free on various webstites.
Furthermore as >>810035 said, by watching the anime, you will now of a good manga, that you would otherwise have missed out. There are also a few cases where the manga hasn't been fully translated, and a new show based on the manga, will create the necessary demand in order to translate it from where it left out(see Killing Bites, and to some extend Warau Salesman).
Lastly for shows that you know will adapt the entire manga, it's a question of low time preference vs high time preference, as in when the anime reaches the manga, and it either ends waiting for new chapters to appear, or it starts to create filler content, do you either wait for the new season to start(high time) in the idea that the animation, dialog and music are good and make the source material better and the wait is worth it, or you read the manga now(low time) and enjoy the story now(or at least, once every week, instead of once every year)? Of course you can also do both.
>the second season will be utter trash because it was made to scam the already existing fanbase of their money.
That can be true for manga as well, so I don't know what you are getting at. The author, instead of ending the manga here and now, decides to milk it for extra sheckels, like with Attack on Titan.