Buying in bulk saves, period. Yes, you may shell more out-of-pocket at the time of purchase when buying in bulk, but the savings are realized almost immediately... Fewer trips to the store, the price savings per pound, the quantity of the bulk purchase over repeated, smaller buys.....
Take American cheese for example. A package of Kraft Singles, 24 slices (16 oz) typically costs around $4. That's 16.6 cents per slice. But buy in bulk, and you can have those slices for just 10 cents a slice. Pictured right, a 5 pound block of Kirkland (Costco) brand American cheese slices, 120 count. I bought it for $12. All I have to do is portion the slices out and I've got sliced cheese for weeks. That's one less item I have to put on my shopping list for awhile, one less item I have to pick up at the store because I'm out of it. And frankly, this cheese flat out tastes better than the Kraft singles and I don't have to play with that damn singles wrapper!
Still don't think buying in bulk saves or isn't convenient enough to make it worth your while? Do the math. It takes five packages (and most likely five trips to the store) of Kraft Singles costing you $20 to give you the same amount of cheese in the Kirkland brick. Buying in bulk saves you $8!