Even though Bayern Munich does not have anything to play for since they qualified for the Uefa Champions League last 16, Bayern Munich would like to beat Manchester United to protect their pride of not losing to a big side like Manchester United. With the current form of Manchester United, they will find it difficult to beat Bayern Munich.
There is even more than pride here for Bayern to play for, when looking at the rest of the teams on Group A which one out of those teams seems like the one that could become more dangerous if they were to improve during the next weeks and months? And to me Manchester United takes that spot, so instead of just letting them remain on the competition and give them the chance to improve themselves and become a dangerous rival, why not eliminate them on the spot by beating them now that they are not playing well?