Requiring users to make a specific number of posts is really needed. A user making only 5 posts per week is not helpful. Usually, even if you don't make the required posts for 1 week, you won't be removed from the campaign.
For avoiding spams managers usually check the posts history before accepting participants and that's why the person who has made low number of posts before applying for the campaign is not accpeted.

Then the participant will be likely removed due to burst posting.