Soda has around 100 calories per 8 fluid ounces (varies slightly with type of soda). So you get a 32 ounce drink, that's 400 calories. That's a fair bit, even by fast food standards. Most fast food burgers are in the 800-1200 calorie range (a double quarter pounder with cheese is 740 calories for reference). So you are adding 33-50% more calories to a meal with a 32oz soda.
Well the thing is, the calories in that soda won't do much if anything to fill you up. Drink as much as you like, you still feel hungry. Not so with a hamburger. While it isn't high quality nutrition, it is still plenty of protein, fat, and carbs and your body is going to be satisfied by the consumption of it.
Thus cutting out the soda really can help. You reduce a non-trivial amount of calories and it isn't likely to make you feel less full. Ya, you are still eating fast food and it is not high quality nutrition, and it is high calorie for what you get, but it is better than just drinking sugar water which is more or less what soda is.
Weight loss and eating healthy isn't an all or nothing proposition. There is better and worse, and cutting out soda is doing better than leaving it in.