He looked good early in the season playing high up the field, he looked like the perfect foil for Messi. Add to that he had a bit more aggression going forward than other Barca players, and he looked like the ideal candidate to make them that bit more unpredictable. With Villa injured, and Alexis not bedded in, Fabregas did a good job.
In terms of how Fabregas fits, Iniesta doesn't score enough goals to do what Fabregas was doing going forward. Playing Iniesta and Messi in the middle, in tight games where Messi drops back to find space, doesn't create as many chances as Iniesta going wide.