Often when tenants lose keys to a deadbolt, a shady landlord or superintendent will say the knob lock is “good enough” to avoid paying to have the deadbolt replaced. Consequently, tenants might install their own deadbolts.

You might ask, why not just replace the existing lock!? Well, the super is going to call a locksmith to do that, and the locksmith can charge more for installing a new lock rather than replacing the old one. Plus, an extra lock when everyone’s is home for the evening == extra security.