In short
The occupancy permit, formally the building use permit, is the document showing that a building has been completed in line with its project and with legislation. It is obtained from the municipality and formally registers the building as ready for occupation.
Without it a building is not legally considered complete. Making electricity, water and gas subscriptions permanent, converting construction servitude into freehold title, and certain lending processes all depend on this document.
Living in a building without an occupancy permit may be possible in practice, but the issue surfaces at the moment of sale. A unit without one appeals to a narrower pool of buyers than one with it, and the price reflects that gap.
Why it matters to an investor
One of the clauses to look for when buying off-plan is whose responsibility the occupancy permit is and by what date it will be obtained. The delivery date and the permit date are not the same thing; both should be written separately in the contract.