The purpose for the Emancipation Proclamation was to keep Britain from supporting the South.
Britain looked forward to an independent Confederate nation with whom to trade for cotton. The North had stiff tariffs on cotton, which financially hurt both the Southern states and Britain — the South sold less cotton than they would without the tariffs, and Britain would have paid lower costs for the cotton.
The enormous success of General Lee’s armies early in the war emboldened Britain to ally with the Confederacy and offer supplies for them at the very least, and possibly naval support.
Great Britain had just abolished slavery in 1833 - just one generation before.
The Emancipation Proclamation succeeded in pitting British public opinion against the slave holding south, and prevented the British from supporting the South with arms, munitions, material and naval support.
Cotton was an export. Tariffs are placed on imports.