It's become a lot more common over the past few years (likely due to the change in the economy) for meeting planners to request break, beverage and snack items on a consumption-basis. Only recently did I realize that some seasoned meeting planners are barely aware that it's an option. Some of the confusion may lie in what can be offered on consumption and what cannot, and how the charges are made. Ordering on consumption can be your most economical option in many cases. In others, it could be detrimental to your budgeting.
Let's say that the last day of your program at XYZ hotel runs until late in the day, but many of your attendees may trickle out early. You want to offer an afternoon coffee break but have no idea how many people will actually consume from it. This is a great time to offer items on consumption, so that you are only charged for what is consumed. On the other hand, during a short 15 minute morning break at XYZ hotel with no in-house coffee kiosk or nearby cafe, you can be confident that the majority of your guests will tackle the coffee station and morning treats, every single one of them. So you may as well pay a per-person cost rather than a per-item cost. I could go on and on with further scenarios, but this is the general idea behind the decision in many instances.
So you ask, what items can and cannot be offered on a consumption basis? Non-perishable items can almost always be ordered on consumption. The hotel will order what is guesstimated to be an accurate amount of the item and display it accordingly on your break, replenishing as necessary. I said "almost always" because you have to take into account the hotel's general offerings on a regular basis. If you want to charge on consumption for an item that the hotel will never use again (i.e. wasabi peas?), you might receive a little pushback on the hotel's end, for good reason. Perishable items like yogurt and fruit are commonly requested on consumption, but these perishable goods will only spoil before the hotel can use them again.
Bottled juices, water and sodas are great items to set up for consumption, as is coffee. The trouble with coffee is that you pay for what is brewed, not for exactly what is consumed. If a half gallon is left over at the end of the day, you'll still be charged for the full gallon because it was made and brewed in anticipation of your group's numbers, and that coffee cannot be re-served.