Why did lincoln write the emancipation proclamation?

He wanted to change the federal legal status of more than 3 million enslaved people in the designated areas of the South from "slave" to "free" , although it's immediate effect it was less. it has practical effect that as soon as the slave escaped the Confederate government by running away or through advances of federal troops, the slaves became legally free. It also ended the war because slavery was a big start of the war