Someone could click a link on an ad and be on your “catalog” and able to order with very low friction, rather than seeing an ad, ordering a catalog, getting the catalog, then calling in or mailing an order.

(Also, Amazon was initially specifically in books, and got started just after a few giants like B&N and Borders had crushed local independent bookstores and replaced them with giant, discounting, but more impersonal and on average more distant from the customer big box stores, so they were hitting a market uniquely (or at least particularly) primed for a convenient to access heavily discounting remote seller; not only was the internet a sea change