In lincoln's view, why is it necessary that the union win the war?

It was necessary to win the war because the south wanted to separate and that would make the south and north both weak also the south wanted slave but the north didnt 

Hey there!

For Lincoln, as the President of the United States of America, his main goal in the Civil War was to preserve the Union in order to keep the country united.

For him, preserving the Union was essential to keeping the government of the country a democracy and to prevent any future turmoil from arising due to future conflict from a divided nation. With the North and the South fighting over their beliefs and lifestyles, America was at risk of falling due to the internal conflict among its own people. 

Hope this helps and have a wonderful day!