Serves as “a” predictor is key here. You can have very good predictors and predictors with very lackluster performance. What deviation in rating gives you say only a one percent chance in winning is an indicator for the quality of the predictor. Think of an hypothetical ELO-rating for chess >> tennis >> soccer. If that’s too trivial, perhaps archery is a nice example. I think ELO is applicable to archery. Which rating is a stronger predictor? Where are upsets more likely? Chess or archery.