Preceding the civil war:

Tariff of abominations "aka tariff on imported manufactured goods" this helped the north which was an industrial economy failing to compete with Europe for the southern agricultural equipment needs.

This tariff lead to the nullification crisis which challenged the scope of federal government, states rights, and started southern talks of secession

Morril Tarrif act, once again further raised taxes on imports which strengthened northern jobs while hurting southern commodity producers.

The north was being lobbied by industrialists for a traiff system that could only be imposed by a stronger federal government.

My question is the following. if slavery was the real purpose for the war, why was the emancipation proclamation so significant for military strategy 2 years into the fight?