Rents

You would be sharing a fraction of the total apartment rent irrespective of other flat mates occupying or not occupying the apartment . So, you pay for the piece you are eating and not for the entire cake.

You would be responsible for the entire rent. So in a case where a flat mate sharing the rent with you vacates the apartment, you would be liable to pay his share too. Here, you pay for the entire cake whether you eat it or not.