I'm not sure Iunderstood your question.
The function allows you to store the category so that you can later apply it to other chemicals, provided that they are in the applicability domain. It will always make identical predictions for your target chemical. For read-across categories it will indeed always come up with the same answer, but the answer will be true only if the chemical is in the applicability domain (i.e. the set of rules that you used to build your category).
And that's where the crux lies.
It makes little sense to save a category which is taylored precisely to your target chemical (i.e. has a very small applicability domain).
But for categories which have a wider applicability domain, it might prove to be useful to save the model and reuse it for the prediction of other chemicals.
Don't know if this is helpful. Maybe I misunderstood your question.
You did understand my question correctly and answered it perfectly; thanks.