The strategy was that if the Oda army was stuck in Kitaomi, Nobunaga would not be able to fight against Shingen's troops at full power and this would increase the anti-Nobunaga allied forces' chance of winning the battle.
北近江に織田軍を釘付けにすれば信長は全力をもって信玄の軍勢とぶつかることができず、反信長連合軍の勝機は高まる。