Cold lunches are okay, but after a while they can get a tad monotonous. The ability to warm your food at the office, in the car or anywhere else, not only means you can enjoy a warm lunch, but it also means you have more choices available in terms of what you can eat. If you get some fast food, for example, being able to keep it warm can make the difference between a tasty meal and a disappointing one.
I like this model because you can remove the heating element:
Portable food warmer pictured: HotLogic Mini Personal Portable Oven, blue
If the warmer is going to be used in your car, make sure it has a 12 volt plug, or at least the appropriate adapter for plugging it in the lighter outlet. In some models, the heating element is removable. This is handy since you can then use the warmer as a lunch bag when the element removed. It also makes cleaning easier. Remember that these are called warmers and not heaters. These will not cook anything. They are only meant to keep cooked food warm.