Stock, returns and support break down where your sales channels overlap
A store platform handles checkout well and leaves the work around each order to a stack of add-on apps that rarely agree with each other.
Inventory across channels: your site, marketplaces and wholesale accounts pull from the same shelves, but each keeps its own count, so a fast-selling item oversells somewhere before the counts catch up.
Returns: the request arrives in one place and the package in another, while the refund, restock and exchange are handled by hand, often days apart.
Customer service: to answer a simple question about where an order is, an agent checks the store, the shipping tool, the returns app and the email thread, one tab at a time.
Wholesale orders: buyers send purchase orders by email or spreadsheet, and someone retypes them, checks stock and builds the invoice.