Living in a very urban area, busy suburbs of Washington, DC, I knew that if for some reason I could not get the booster at the local Giant grocery store's pharmacy that there was a CVS just a mile or so away, plus a Walgreen's another mile or so further down the road. I also took action as soon as I saw that Moderna boosters were now available, and in several convenient locations, as that has happened only recently. I figured a lot of people might not yet have become aware that they could get the booster in one of these places, especially Moderna, since it was approved and boosters made available later than Pfizer had been. Eligibility, too, would be factored in, and in my case, given my age, no question, but last I knew, younger people wouldn't be able to do this yet. I wanted to beat the crowd and was happy when everything went so smoothly.
Definitely if someone is living in an area where there is not a pharmacy on every corner and in every grocery store as well, yes, it would be important to go online and book an appointment rather than taking chances on issues coming up and not being able to get the booster when one expected to do so.