Offering a Draw

Would someone explain to me why an opponent would offer a draw when I am one move from checkmate?  I don’t understand.  If you know you are loosing that badly why would you not resign?  But to offer a draw?  Just to improve your rating?  My rating isn’t as high as I would like, but the only way to improve your rating is to improve your playing.  Should I feel guilty because I did not accept the draw?  Any thoughts??