Could you expand on this? It's not clear from the outside how this would be such a central part of the industry.
My guess is this person meant relationships with the farmers and producers being critical. At least this is true for us.
If you want the freshest produce and other local items, then you need to know the farmers and other vendors in your area.
If you want to go mass market instead, then there are only a few wholesalers to choose from.
It's not easy for a small to mid-sized brand to try to get their product into stores when each physical brick and mortar store is a pitch in and of itself. So what ends up happening is brands partner with Agencies who specialize in building relationships with each of the store managers in a region and advocating for all of the brands in their portfolio.
I realize this is a step removed from Instacart and delivery services, but the whole industry tends to operate in a similar interpersonal/relationship-based way.