This feature allows the submitter of a campaign or request (the owner of the track) to have the repost(s) of their track automatically removed after 48 hours.
This is helpful if you feel the number of reposts looks unnaturally high against the number of likes and comments on a track's stats on SoundCloud when it has been promoted with RepostExchange. This feature gives you the option to redress that balance should you wish.
48 hours is a reasonable 'window' in which a reposted track can be expected to pick up engagement. It is the period in which a reposted track will receive the majority (if not all) of the engagement that a given repost will bring, therefore it makes little to no difference to engagement if a repost is removed after 48 hours.