The idea being conveyed in this verse is that all of these groups normally leave something behind. Thieves never take everything because they are in haste at night and so they take only that which is most valuable to them. Grape gatherers who harvest grapes in the field normally leave one or two bunches of grapes behind on the vineyard; they never take absolutely everything. All these groups (thieves, plunderers, grape gatherers) at least leave something behind. But when the Lord comes, he is going to leave nothing behind.
5 If thieves came to you, if plunderers came by night how you have been destroyed! would they not steal only enough for themselves? If grape gatherers came to you, would they not leave gleanings?