According to the Glossary issued by the INSTITUTO NACIONAL DE LAS CUAIn terms of the Tourism, Hospitality and MICE sector, and according to a glossary drawn up by the Instituto Nacional De Las Cualificaciones, belonging to the Dirección General De Formación Profesional, one of the departments of the Secretaría De Estado de Educación, Formación Profesional y Universidades de España, this is the definition.

Zona Noble:
Son aquellas áreas de un hotel que pueden ser utilizadas por todos los clientes y no son habitaciones. Se dividen en zonas de uso común (hall de recepción, pasillos, sala de restaurante, entre otros) y salones para la celebración de eventos.
So, the public and common areas that are not guest floors or the workplaces of staff (kitchen, laundry, etc).

After a tiny bit of research that took all of 3 minutes, I would venture that the origin of this goes back to the times before we had widespread and globalised tourism, so, think great castles, lords and ladies, Downton Abbey, etc.
Historically, the Zonas Nobles of a dwelling are the areas of a house that were used by the main occupants of the house and any visitors they may choose to entertain, as opposed to the domestic staff and servants or other employees. The basement and a certain portion of the lowest floor, and the highest up floor, the attic and the eaves of a building would be where the household staff would work and live.
A fascinating Argentinian dictionary published by Diario de Arquitectura de Clarin, called Diccionario de Arquitectura en la Argentina, further consolidates this concept. Under the definition of COCINA [kitchen] (starting on page 92), we find a mention of “zona noble” (p. 94) to clearly differentiate it from all the areas used by the household staff (i.e.: kitchen), in houses occupied by the gentry (initially) and then later by the up-and-coming bourgeoisie:
“Dentro del esquema clásico que rige la organización en altura de estos tipos de Vivienda [here, the argentine “Casa de Patios”], el plano tierra es donde invariablemente se ubica la cocina. Este primer nivel constituye el basamento de servicios sobre el que se apoya la zona noble de la casa, dando cabida a todas aquellas dependencias involucradas en el abastecimiento y mantenimiento de la misma, y dotándosela por lo general de un acceso independiente.”

It has since evolved to what it is today, a concept used to refer to the parts of a building that are open to the public, visitors, guests, etc., and that are not occupied by people working. I use “working” here, to make this a broader definition, because we can find a “Zona Noble” in somewhere as unwelcoming as a factory or a power plant, or somewhere maybe friendlier like a small, rural hotel. In all cases, it would mean the same: the area of the building or complex that can be construed as accessible to all, unaccompanied by staff and that is not the workplace of an employee.
LIFICACIONES belonging to the DIRECCIÓN GENERAL DE FORMACIÓN PROFESIONAL, which is one of the departments of the SECRETARÍA DE ESTADO DE EDUCACIÓN, FORMACIÓN PROFESIONAL Y UNIVERSIDADES,

Zonas nobles: Son aquellas áreas de un hotel que pueden ser utilizadas por todos los clientes y no son habitaciones. Se dividen en zonas de uso común (hall de recepción, pasillos, sala de restaurante, entre otros) y salones para la celebración de eventos.