On July 19, the Florida Secretary of State filed this brief in Mazzilli v Townsley, the case over whether the August 2012 Democratic primary should be open to non-Democrats, for the office of Miami-Dade County State Attorney. The judge had asked the Secretary of State to enter the case. The case was filed by some voters who want to vote in the Democratic primary for State Attorney, even though they are not registered Democrats.
If this were in State court, then Florida would have to get the change precleared. So it appears that the reason for going to a federal court to interpret Florida laws is to circumvent the USDOJ.