Members of Congress sometimes need “cover” when they vote in ways that are likely to be unpopular with their constituents.  The “cover” gives them an excuse for voting as they did.  When members of Congress need this kind of cover, they often turn to their party leadership.  They may be able to say that they voted in a particular way against their own will because they felt they needed to go along with the party for the greater good.  If they can find an interest group that has credibility with their constituents and which supports their vote, they can use that as cover too.  Cover, then, is needed to provide an excuse for an unpopular vote.