Why would the north want to capture atlanta?

It did not want to, according to Grant's strategy, which was simply to destroy the two armies under Johnston and Lee.

But when this still hadn't happened, month after month, and Lincoln's credibility was about to be tested in the 1864 Election, the capture of Atlanta was seen as the kind of victory that would reassure the Northern public.

It did just that, and Lincoln crucially won his second term.