That works up to a point but sometimes you need something very particular where the substitutes available are inferior to the point of being wholly undesireable

That works up to a point but sometimes you need something very particular where the substitutes available are inferior to the point of being wholly undesireable (like substituting hot italian sausage made in-store for prepackaged processed sausage filled with onlu god knows what) or niche enough that stores typically only stock 1 option (like clear, unflavored, unsweetened gelatin). Missing either of those items is a deal breaker when you're planning for specific meals or dishes you plan to bring to an event.