It narrows the candidates down to 2 feasible choices, giving the voter an easier comparison, less research needed to keep the voter informed than when a voter would have to compare a larger group of candidates. It also avoids the irrationality of Arrow's Voting Paradox or Impossibility Theorem.
but compared to parliamentary systems, it gives less voice to smaller minority groups who may form 3rd parites and have some leverage when neither of the two major parties have a majority. Coalition governments formed then will give a 3rd party better representation. Also 2 majority parties, if they collude with a effective duopoly on some issues such as financial, have no check on their power or ability to enrich themselves at the expense of taxpayers.
