Why swich dog food slowly?

thats the most stupidest thing i have ever hurd. i need to switch my dog's food slowly so he wont have dierrea. i feel like the company who tells you this is selling you crab for your dog. thats like saying if im eating chicken one day and want to eat pork another i need a little bit of chicken to go with my pork. something is wrong with are dog food. i did some research and theres one brand that doesn't need you to switch slowly and that Bil-Jac i herd a lot of good things and a lot of bad things. what you guys think