What is the effect of choosing the wrong topic(s) during conference paper submission? My domain is Image processing.

While submitting papers to conferences, authors are usually asked to select a primary and secondary topic for there paper. 


What is the actual purpose of this?
What if authors by mistake select a wrong (not completely) topic instead of a more suitable topic? Does this get changed before/during the review process?
What if this is not corrected and paper is assigned to a reviewer who thinks this is not the proper topic of the paper? Does this affect the paper review process or increase chances of rejection?
Yes, this helps organize the papers for the reviewers and structure the conference. Ticking the wrong box can indeed cause a paper to be rejected, as not all reviewers read all of the papers and realize that it is just misplaces. Do choose these with care.