"Rules" just stores the rules in the same way they were entered as an input to WolframModel:
In[] := WolframModel[<|"PatternRules" ->
{{a_}} :> {{a + 1}, {a - 1}, {{a + 2, a - 2}}}|>,
{{1}}, 1]["Rules"]
Out[] = <|"PatternRules" -> {{a_}} :> {{a + 1}, {a - 1}, {{a + 2, a - 2}}}|>This is useful for display in the information box of the evolution object, and if one needs to reproduce an evolution object, the input for which is no longer available.