Why do some conferences ask authors whether a paper they are submitting to the conference was published in arXiv? Why do some conferences ask authors whether a paper they are submitting to the conference was published in arXiv?

Example:


  Submitted elsewhere or arxiv? Is the paper being submitted, or has it been submitted to another conference/workshop? If so, put the name of the other conference/workshop in the box below. If your paper is on arxiv.org or similar place please mention this here.
Presumably some reviewers check for plagiarism, so it is useful for them to know that there exists a preprint out there that is expected to significantly overlap with the submission they are reviewing. 

From the CFP for the conference you gave as an example:


  Reviewers will be told: “The author(s) have notified us that there exists a non-archival previous version of this paper with significantly overlapping text. We have approved submission under these circumstances, but to preserve the spirit of blind review, the current submission does not reference the non-archival version.” Reviewers are free to do what they like with this information.