From reading your thoughts on GM'ing over the years I've developed the impression your "ideal gameplay" runs quite close to freeform narration. In what ways do you find rules/systems to be useful?
i think your impression is pretty close to correct! my first answer is that a good rule or system eases the parts of that which otherwise bog down the play act. for example i think the model i hold in my head for moment to moment play - someone makes a claim, someone responds with a counterclaim, discrepencies are then conversationally addressed - is served well by rules that ease the difficulty of adjudicating a claim relating to things of such complex scope they would be dragged to a halt by talking them tjrough naturalistically, for example travel over a great distance (in which i am likely to over or underestimate to the detriment of my later attempt to cohere what all the consequences of travel were), or the outcome of an action in which none at the players at the table have expertise irl.
my second answer is that im generally amenable to rules and systems that interject a complication to interpret so long as they are not overly intrusive to the play from that result. i think this is a combination of the mechanical complexity/prescriptiveness of the rule itself and also connected in many cases to the prose surrounding it to describe it - the latter is especially a big deal in pbta games where the mechanical is so often prosaically written.