However, the Sagrada Familia is already a massive benefit for the city due to the visitors it attracts. Its administrators could've just raised the middle finger and Barcelona would definitely not have tore it down nor stopped its construction.
Why did they enter the agreement then? The Sagrada Familia's original plans include a number of gardens that span outside of its current land. With this agreement in place, now the city can officially start making plans to eventually evict residents in the affected land space, freeing it up for the monument's gardens.