Anderson was very popular as a supporting character, obviously—of all the supporting characters in Dredd’s stories she was the one who most seemed like a natural to spin off into her own feature. (It may be less that she’s a great character on her own than that she’s a great foil for Dredd: irreverent, inexact, emotional, and totally on the same page as he is.) Still, it’s not clear to me as a reader why it initially seemed like having a second Judge-based series in the weekly was a good idea. It might have been a way of dealing with reader demands for more Dark Judges and more Anderson without having to do another “Dredd fights Judge Death” story—the stakes for the first two were pitched high enough that it couldn’t have been easy to find another angle.
The question Alan Grant (and sometimes John Wagner) faced after that, though, was what kinds of stories it was possible to tell with her that it wasn’t possible to tell in the context of Dredd’s own series. Supernatural stuff, obviously, and squishy psychic phenomena, since those are hard to square up with the Dredd premise of “contemporary American cultural trends taken to outlandish extremes in a sci-fi context.” (But that means they’re also hard to square up with the Mega-City One setting.)
One other answer was that Judge Anderson could simply act as a second channel for Judge Dredd: adding to the backstory and moving characters into position. Anderson’s first full-on serial, “Four Dark Judges,” followed up on “Judge Death Lives,” and established a basis for future Judge Death plots; “The Hour of the Wolf” set up the idea that the Apocalypse War wasn’t so much the conclusive end of a cold war as the instigating event of a very long chain of resentment and revenge.