house brand - because of the obligation to sell lower than 90% of the competitive brands tend to be of different quality. NTUC has to sell like 5% lower than the next competing brand, obviously, they will squeeze more from the suppliers - who in turn will have to work with whatever size of cloth left for them.
So, it is not entirely true to say the quality is the same. Of course, for food items, they have to meet the basic requirements for food safety.
Suppliers are generally willing to produce house brands for NTUC (or other supermarkets) because they keep the machines occupied and at least provide confirmed turnover (not necessary profit) - maybe sufficient to cover costs or make marginal profits. Or to keep the production line going.
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NTUC will represent a big purchaser - with dedicated purchasing department and perhaps buyers to look after each category of goods. You can imagine the amount of paper work each buyer have to go through if they deal with individual importers / manufacturers. Purchase orders, delivery notes, bills, invoices, etc has to be transmitted, checked for each transaction. Even with technology to help in e-paper them, it is still hard and tedious work. Hence, many like to deal with one or few wholesalers who on their ends deal with many / more importers or sellers. In doing, they congregate their purchasing amount (and bargaining power).
Also, sometimes, they aggregate their purchases and consolidate shipping to save costs for example, a couple of containers per day from China / Malaysia - aggregated by their wholesaler.
But of course, certain items due to contractual obligations or nature of product, they have to deal directly with the manufacturer for example, I reckon they will buy tiger beer directly from APB and not through intermediaries.