There was speculation at the time of the episodes original airing that Darla wasn't Darla at all, but was in fact The First. Jasmine's plan of world peace would have put a HUGE kink in The First's plan, so by talking Connor out of what Cordelia/Jasmine wanted him to do, it would keep The First's plan of moving past Sunnydale alive and put the skids on Jasmine's plan at the same time.
However, this was never confirmed or denied, so we just don't know.
Personally, as a Connor fan, I like to think that it was the redeemed Darla being granted the opportunity to return to try to save her son. After all, she did refer to him as the one good thing she/they did, and for him to be heading in the direction he was heading must have been hard for her to watch, if she was in fact working/with the PTB's.