Two issues:

(1) Subletting by a rent-controlled tenant to a sub-tenant, for a profit and without the consent of the landlord.

(2) “Giving” a rent-controlled apartment to a friend or family member, without the consent of the landlord.

A rent-controlled apartment that a tenant has held for 20+ years and is renting at a small fraction of the market rate is a valuable asset that some tenants want to capitalize on.