The Rent Ordinance defines housing services to include quiet enjoyment of the premises, repairs, replacement, maintenance, painting, light, heat, water, elevator service, laundry facilities and privileges, janitor service, refuse removal, furnishings, telephone, parking, rights permitted the tenant by agreement, including the right to have a specific number of occupants, whether express or implied, and whether or not the agreement prohibits subletting and/or assignment, garage facilities, parking facilities, driveways, storage spaces, laundry rooms, decks, patios, or gardens on the same lot, or kitchen facilities or lobbies in single room occupancy (SRO) hotels, supplied in connection with the use or occupancy of a unit.
While some of these items seem inexorably intertwined with the rental unit itself, others – like access to a parking space – could previously have been “severed” by simply changing the terms of the tenancy to no longer include a parking space. The Board of Supervisors worried that this led to de facto evictions, where landlords gradually took away the amenities that tenants depended on, in an effort to urge them to vacate. By requiring “just cause” for severance, the City prevents these items from being removed piecemeal, where a landlord may only take away such housing services with sufficient cause (e.g., permitted work to convert garage space into an ADU unit).
The corollary of this concept is found in Rent Board Rules & Regulations §12.20, which provides that a tenant cannot generally be evicted for violating a unilaterally-imposed change to the term of their tenancy. (This was the subject of the 2015 opinion Foster v. Britton.)
You can read the full text of Ordinance 178-06 here.