@ispcizero3rd well, pls first make clear what you mean by novel promotion
Authors typically begin from writing WN in a free platform like syosetu. They write for free and people can read it for free.
If a publisher is interested, they'll contact the author and then the story will be reviewed and edited by the editor, sometimes they even alter the story so the WN reader will be interested in buying the LN.
Manga is next, and will follow the LN storyline.
And if possible anime is after that.
Whether it's manga or anime, of course they serve to promote the story to a larger audience.
However, if you mean that this manga is just like an incomplete "bait", then the answer is big
NO.
The continuation of the manga depends on the manga publisher and its popularity.
And there's surely a good reason to why they only choose to do the first and second arc of the story.
For the rest of the story...you can read it by yourself 😊