Commercial real estate is often leased as an empty shell - while it is true that you could probably find a storefront already outfitted as ice cream store, or at least for foodservice, the tenant is probably still going to need to bring all kinds of fixtures, branding, etc. I'm not sure how the line is drawn between whether you need an architect for that or some other kind of designer, but point is that leasing a space which is habitable but not ready for commercial use is common in the US. Commercial tenants often get really broad license to customize their entire space within the confines of the walls.

You still have to submit detailed plans to the city for your commercial space. An architect is best able to help here as they are familiar with the building code, permitting process, etc.

They can also help account for things the prospective ice cream shop owner isn't aware of. For example, regulations around hand-washing stations for food-based businesses.

I happen have an ice cream shop and we used an architect. Their experience designing other retail locations was very helpful.

I'm pretty sure that a random commercial space won't be "ready to go" with ample deep freeze in the back and a counter with freezers in the front. Sounds like custom cabnitry, lots of electrical work, probably some ventilation... putting all that stuff together in a pleasing arrangement is definitely the work of an architect. They don't just do freestanding buildings.