you need adequate calories(energy) to sustain normal bodily functions (hormones, organs, thryoid, etc) as well as metabolism. If you are not getting enough you risk drastically decreasing the output of the above mentioned. IE fatloss will hault, you feel like *crap*, loss of muscle, etc. when you go back to eating more you will regain what you had primarly lost and perhaps then some.
its about finding the right balance. thats why we stress either carb cycling(calories basically) and refeeds while dieting, to keep these above things running efficently and not slowing fatloss as well.