They didn't need it just as much , they had all the industry , at the start of the war the North had the most money , arms , equipment , etc the North was doing fine it's economy wasn't dependent on slavery the South's was
The South did not need the North at all, hence the secession. The problem was that Southern Money was held in Northern banks at the time and getting that revenue back to its rightful owners, was near impossible. The South was planning on to rely on exports to England in exchange for Ships and Munitions. The North shut this down with a Blockade as they knew the South would have made off perfectly fine with just England as a trading partner.
Also, the North did not have the money to fight the war, they should have, considering their Industry and sheer population numbers when compared to the South, buy they were strapped for any cash. So what did Lincoln do? First he enacted a massive bond measure that forced citizens and Business to buy into the war. That failed to produce enough revenue. So what was next, Salmon P Chase, the Secretary of the Treasury then decided to print more and more money in the form of Greenbacks and forced the citizens to accept them for all debts.
That still wasn`t enough, so The Bureau of Internal Revenue was created (later to be known as the IRS) in which everyone was forced to pay income tax. That devil still exists today.
The North barely made out a few times and the South could have easily taken DC several times, especially when McClellan was purposely bumbling around on the battlefield because he really didn`t care for Lincoln.