It’s often done to make sure the reader tries to weigh the merits of the content by itself.
My understanding is that it’s usually meant to serve the opposite purpose: to alert readers to the possibility of bias so they can evaluate the content with that in mind and decide for themselves whether they think bias has creeped in. The alternative is people being alerted to the CoI in the comments and being angry the quite relevant information being kept from them, not that they would otherwise still know about the bias and not be able to evaluate the article well because of it.
My understanding is that it’s usually meant to serve the opposite purpose: to alert readers to the possibility of bias so they can evaluate the content with that in mind and decide for themselves whether they think bias has creeped in. The alternative is people being alerted to the CoI in the comments and being angry the quite relevant information being kept from them, not that they would otherwise still know about the bias and not be able to evaluate the article well because of it.