You have a big road trip. Knowing that you are going to be driving all day, you take your vehicle to the gas station. After you select the proper gas, you fill it up to the top, because you need it.
You wouldn't put rocket fuel or diesel in your two door hatchback. You wouldn't put low cost premium in your V8 hemi-truck. Nor would you only fill up the tank 1/4th of the way for your 8 hour drive.
Say you only have a 4 hour drive but you fueled your car for an 8 hour drive. At the end of the day you park and go to sleep. In the morning your tank is magically empty and your car has another piece of luggage tied on top! It gained weight, and you lost your extra fuel. Bad fueling choice...
Your body is no different!