I think that was the purpose why Roma sign back Ranieri, his task is to save Roma from relegation zone and Roma know Ranieri won't able to make Roma to get trophies. Gasperini is better than Ranieri, he can brought Atalanta to won Europa League, as we know Atalanta is just a mediocre team in Serie A, although it took a long time for Gasperini to get trophy.
Who knows Gasperini might able to bring a trophy for Roma.
If i remember correctly, Claudio Ranieri is a special advisor for Roma and not a coach. Let's also say he will be a safe haven in case something happens or a coach is fired, as has already happened.
Let's say that he now feels he's finished with his coaching career so this is the only solution.