calories are like gas for your car. nutrients are more like wiper fluid, oil, wipers, drive shafts, steering wheel and column, pistons, spark plugs, tires and that ilk. while you need the nutrients there, if you don't have the energy to make it go then it's not going to perform the function that makes it what it is. vitamin a and fiber aren't a fuel that your body can use to make your heart pump and circulate your blood and all of those other fun things that keeps your body alive and working.
while 1200 might be a nutrient minimum, many people need more calories than that to perform those basic body functions. when your body doesn't get the calories it needs, it starts to make cuts. which means that since fat doesn't burn any calories, but muscle does, when there are not enough calories coming in to cover your expenditures [and this is a chronic, over time thing, not a one or two days here or there], you start to burn muscle. the less muscle you have, the fewer the calories you burn. think of your basic monthly expenditures. if you make 4000 a month, having rent of 1000 isn't a big deal. but if you were only bringing in 1500 a month, that 1000 a month would be a bigger burden. and if you suddenly dropped to that 1500 a month and your biggest expense is rent, one of the first things you would look into was finding a place with lower rent. your body does the same. when the intake is so far below the output, the output has to go down. and we really have no idea what corners your body is cutting to reduce the amount of that deficit.
now when you have a lot of weight to lose, having a larger deficit between input and output is fine. you have a giant calorie bank in all that stored fat. and the more weight you lose, the less energy you spend hauling it around, so even if you were still eating the same amount as when you started. your calories deficit has decreased.
but if you exercise more then that bumps up your output, so you need to eat more so that you maintain that deficit in that sweet spot. because if you cut too many calories, then your body starts doing those funky expenditure saving measures. and at the extreme end of that is that people who have been put on starvation diets tend to put back on all the weight they lost plus about ten percent if they have the opportunity.
-google first. ask questions later.