I asked that specific question from a friend of mine.

His answer was:

* we charge $20 for a plate that costs is $4 in ingredients because we are lazy to squeeze it down to $3.50.

* Single sourcing means I don't have to trust ten of my employees with credit cards to run around the city to get stuff.

* Single delivery means I can schedule it better.

Retail packaging is a pain to deal with, take for example salt. You usually buy it in small boxes, about 1-2lbs each, commercial bags are 40-50lbs. So you need to buy 20-30 boxes a week at your local store, open the boxes one by one and empty it all into your restaurant bin. It's really a convenience thing, it's much easier to handle, transport and store a single really big bag. You still buy retail for stuff that makes sense though, maybe you only need like 2lb of lemons a week to make a sauce, in that case you'd go retail.