I would define it as simply number of items per distance traveled. How much you get from a location X is a function of how often you are willing to travel to location X. As the cost of travel goes down, your willingness to increase frequency increases.

Food spoilage does come into play and creates a lower limit on the frequency. At some point you'll buy so much you can neither store it nor consume it before it spoils. At some other point on the other side of the spectrum - the cost to travel is not worth the convenience of buying exactly what you want each day. All in all, people would indeed prefer to go more frequently for less, but there are reasons to not do so.

Thus, high frequency does give flexibility, and that is really important. It's a big reason why people like to have their own car, and do things like buy one weeks of groceries and not a full months at a time. That same flexibility is why people rationally choose to make multiple trips when they could make fewer. I do find it quite interesting to think about the various 'sweet spots' in these equations, and the various constraining factors.