You want a muscle to get stronger and hypertrophy (get bigger), by all means, target it. Fat doesn't work quite the same though. Imagine your body as an incinerator. You can't put something in it and decide what will burn first or fastest. With that being said, doing 100 crunches isn't only going to burn fat off your midsection. That simply isn't how metabolism works.
Still not understanding? Last time I checked you couldn't decide where the fat went to begin with. You must lose body fat overall through resistance training, cardio, and nutrition.
But, why? It's easy. The higher you get your heart rate, the harder your body is working and therefore, burning calories. Spot reduction exercises; crunches, leg lifts, triceps dips, etc simply don't get the heart pumping and therefore they don't burn many calories. You would benefit more from a 10 minute run than 10 minutes of crunches any day. Then there is also the whole not being able to choose where the fat burns from problem.
Bummed out? Just think, now you can stop doing 100 sit ups or leg lifts a day. Hooray! Find balance, lose weight, and then start building muscle to get the physique you want. Creating a plan that incorporates cardio, large and small muscle resistance training, and healthy eating habits are your best bet. So do your squats, don't skip a run, never neglect your supporting muscles, and don't eat crap.