So you and your roommate found the perfect apartment, signed the lease and then moved in.
Life is gooooood.
Suppose your roommate moves out and leaves you holding the proverbial bag - the lease. Roommates make lots of agreements about how the rent will be split, occupying rooms and the sharing of chores. Your landlord is not bound by your agreements. Many roommates fashion a written agreement to clarify the really important issues. So if your roommate does skip out on you and leaves you paying the total rent then a written agreement could help your recoup your expenses from the defaulting roommate.