If two stores sell the same items, why can the one that routes shoppers past more departments generate more add-on purchases?

Answer: More product visibility

Easier browsing flow — Easier browsing flow is tempting because a sensible route can make shopping feel less effortful. But ease alone does not add new candidates to the basket. The add-on first has to become visible enough to be considered.

More product visibility ✓ — More visibility gives each product more chances to enter consideration, even when it was not on the list. Retail layout research treats visibility along complete shopper paths as a sales lever. IKEA's routed showroom and marketplace are a concrete version of that logic.

Longer walking distance — Longer walking distance is plausible because passing more departments usually means covering more ground. But distance by itself is only effort. It matters commercially when that extra path puts more products in front of the shopper.