Why did lincoln decide to free the slaves before the war ended

Partly for propaganda and foreign-relations purposes. He was worried that the U.K. might recognize and aid the Confederacy and thought that by making the war look more like a moral crusade against slavery, he could make British public opinion more supportive of the Union. 

Note that Lincoln himself did not free all the slaves; and at first, his Emancipation Proclamation didn't free anybody, in practical terms.