Because it draws the line between what a concierge can do legally in its own name and what it can do on the owner's behalf. A concierge without a real estate licence (carte professionnelle) can perfectly well look after the guests, the housekeeping, the linen — and even collect rental payments or handle security deposits — but only as a service provider to the owner, who remains the legal landlord and sole responsible party.
However, signing a rental contract in the owner's name, collecting rental payments on their behalf into a dedicated escrow account, or publishing listings under its own name: these acts fall under the French Real Estate Act (the Hoguet Law) and require a professional licence, a financial guarantee and professional indemnity insurance.
TheGoodBnb operates under Régine Villedieu Immobilier — a high-street agency in Cherbourg since 2006 that holds all of these authorisations. That's a significant legal safeguard you won't find with most concierge services on the market.