Stated rationale
Vertical integration can be a highly important strategy, but it is notoriously difficult to implement successfully and - when it turns out to be the wrong strategy - costly to fix.
Source: mckinsey.com
What that deal would achieve:
Source: Cliffs-AK-Steel-Investor-Presentation
I would not delve much into these points as they have already been covered times and times again. The sole argument for vertical integration does not mean much unless it solidifies a company's competitive advantages and comes at a reasonable price.
Some stated arguments in favour of the deal, such as guaranteeing demand