You will generally see this when dealing with liquidated or clearance items. As ErseErseInc stated, more than likely they are store returns..
Keep in mind that you will normally see two listings, 'New-with tags' and 'New-without tags'.
Some buyers, expecially those buying designer clothing goods will want the tags in place to ensure authenticity (not that tags really prove that, as they are as easily replicated as are the clothes). Others are just looking for a deal and don't really care...Normally the items with the tags in place will command a higher selling price than the ones with no tags...