Let me preface this post by saying that I am not writing this as a warning or an expose.  I am just writing this as an informative piece on the business that is running a professional kitchen.  I just want to provide a little insight into how things work behind the scenes give everyone a little peek.  So, with that being said, I want to address leftovers.  On more than one occasion, I have been asked by a patron at many of the places I have worked at what exactly happens to all the food that is leftover at the end of the day.  The truth is that the answer is not cut and dry.  There are a variety of things that can, and do, happen to leftover food depending on many factors. 

At the end of the day, most kitchens are part of a business that is trying to make money.  One of the biggest expenses that a  culinary operation has is that of food cost and anyone running a kitchen is always trying to keep that food cost down.  A chef will try to take advantage and squeeze out as much production out of all food products purchased.  A very easy way to do that is to use up leftovers as much as possible and in as many creative ways as possible. Now when I say use up leftovers there are a couple of things you must understand.  A good chef will never sacrifice safety or quality for the sake making money.   The leftovers being used must be up to the standards of anything that would be served as if it were being prepared that day.  Those leftovers must also be safe to consume.  All decent establishments with a good record of safety and sanitation will have a use by date for their leftovers (usually 3-4 days).  If leftovers don't meet these quality or safety standards they will not be used and will be thrown out.

When leftovers are being used the easiest way to use them is to reheat and serve them as is.  However there are a few problems with doing this.  First of all you have the problem of possibly compromising the quality of the product upon reheating.  Proteins have a tendency to dry out when reheated.  Vegetables tend to overcook and just not look as appetizing.  So if the plan is to just reheat and serve, special care is taken to reheat so as to not compromise that quality.  The second problem with just reheating and serving is that someone might recognize the food, especially if the customer base are daily repeat customers such as at a cafeteria or a dining hall.  Some observant guest might recognize the meatloaf you are serving today as the same one you had yesterday. A great way to avoid this predicament is to hold off on serving those leftovers and/or serve them at another station. 
[table][tr][td][/td][/tr][tr][td]French toast that was reheated and served the next day.[/td][/tr][/table]
Of course the easiest way to avoid those pitfalls of using leftovers is to just reappropriate them and use them for something that was not their original use.  Soups are probably the most common way to do this.   Soups are cheap to make and very versatile.  They can use up most leftovers, including vegetables that normally would not be good for anything else do to the poor quality that they were in.  Chicken or beef that would dry out of reheated can easily be put into a soup.  Leftover rice or pasta also works well in a soup.   That soup of the day you love so much at your local eatery is probably made mostly from leftovers.
[table][tr][td][/td][/tr][tr][td]Leftovers make great soup.[/td][/tr][/table]
Soups aren't  the only way to use leftovers.  Another common destination for leftovers, especially meats, is the deli or as a sandwich filling in general.  Most of the time when assembling a sandwich it will be served cold and if it is not, there will be enough accompaniments that if the meat is a little dry it will not be too noticeable.  Sandwiches can also use up any unused sauces or cheeses from different dishes.  Sandwiches are just a great way to disguise leftovers as a completely different thing. 

[table][tr][td][/td][/tr][tr][td]Leftover Pork Loin[/td][/tr][/table][table][tr][td][/td][/tr][tr][td]Turned into Cuban sandwiches[/td][/tr][/table]
The truth is that leftovers can and are used all over an operation. The way the chef sees it, it is just free money, or at the very least money has already been spent and can still be taken advantage of.  It is just a matter of being creative and finding different ways to use up what is already there.  I have seen leftover grilled salmon mixed with capers, lemon juice, and mayonnaise and turned into a salmon salad that was then served from the salad bar.  I have seen unused meatballs chopped up and then added to a meat lovers pizza.  I recently ordered some seafood enchiladas from a restaurant.  When I asked the waiter what seafood was in them he told me a little bit of all the different seafood they served.  To me that indicated that more than likely they were just using leftovers to make the enchiladas.  But ultimately that is ok because I know that as long as the quality of the food was still good (they were quite tasty) and my safety was not at risk then the restaurant is just trying to be profitable.  Really that is what using leftovers is all about. Kitchens are just trying to use up and take advantage of what they have on hand. 
[table][tr][td][/td][/tr][tr][td]Pretty sure these were made with leftovers.[/td][/tr][/table]