The process of making wine falls roughly into two halves : growing grapes, or viticulture, and making wine from them, vinification.
In the past these roles were usually performed by two different people.
The grower would sell his grapes to a négociant (wine merchant) who would vinify them, blend them with the wines of other growers from the same area, and sell them under his own label.
Even if the grower made wine himself, he would still sell it on to a négociant to be blended and bottled.
A good négociant was a specialist winemaker freed from the burdens of growing his own grapes and of having to use them.
If a particular grower has a bad year, he can simply buy elsewhere. And by skillful blending a talented a négociant can produce a wine greater than the sum of its parts.
But this is rare. The aim of most blending is volume and consistency rather than quality. It is also makes it very hard to trace exactly what is in the wine.
There is nothing to stop an unscrupulous négociant from lying about what is in the bottle, and nothing did stop them. Such abuses are notorious.
Many wines are still produced in this way, including almost all champagnes, but the days of the négociant's monopoly are over.
The best growers have realised it is in their interests to make their own wine and sell it under their own name.
It is also in the interests of the customer, since he can be surer of exactly what he is getting. Almost all natural wines are made in this way, mostly by a winemaker and his family on their own small vineyard.
Grower's wines in general are more likely to be natural. Small growers simply can't afford the machinery required to mass-produce and manipulate wine.
A winemaker who doesn't grow grapes is like the editor of somebody else's book. He feels he has to change something in order to show that he is earning his money.
A winemaker who is also a grower is much less likely to mess around with grapes he has spent all year painstakingly bringing to perfection.