A question clients often ask us is why all tenants have to be on the lease. Having everyone on the lease not only benefits the landlord, but it benefits you as well.
Landlords want everyone who will be living in an apartment to be on the lease so they can properly screen them and so they will have more people responsible for paying rent.
It benefits you, as the tenant, to have all roommates signing the lease because then if anything goes wrong, they will be legally contracted to be responsible for their portion of the rent. However, the landlord doesn't care about who pays what portion of the rent, they just want the entire amount. Unless you are on a lease where bedrooms are rented individually, you are ultimately responsible for paying the rent, even if your roommate skips out.
Even if you continue paying your portion of the rent, if your roommate does not pay their part or if they move out, the landlord can still take steps up to and including eviction to get all of the rent. So when choosing your roommate, pick someone who you can get along with and who you know can be responsible in financial matters.