The classification system is run by the Instituto dos Vinhos do Douro e Porto (IVDP) and shares some similarities to the classification of Champagne vineyards in that one of the purposes of the system is to ensure that vineyards producing grapes with the highest quality potential receive a high price.
[1] The origins of this system dates back to the quality control regulations installed by Sebastião José de Carvalho e Melo, Marquis of Pombal in the mid 18th century.
[2] One of these powers was establishing licenses for growers of Port wine grapes and dictating limits or "quotas" for what could be harvested.
[3] These measures were enacted to ensure that the market wasn't flooded with Port and that the wines that were available were of high quality and not something that could tarnish the reputation of the Douro.
A quinta's rating will dictate the permitted production level of the vineyard and what price the grower can receive for the wine they produce from the grapes.